Yes there was. The risk was that if there was life on the Moon, returned to Earth, it could in the worst case harm us and our ecosystem. Joshua Lederberg, who got his Nobel prize for his work on microbial genetics, was first to draw our attention to such issues. He was already concerned about it back in the 1950s. Carl Sagan was another early pioneer, who publicized the issues in his popular books.

It is still an issue, no longer for the Moon, but for Mars, Europa, Enceladus, anywhere that might have life. Jim Rummel, former NASA planetary protection officer, put it like this in his foreword to Michael Meltzer's "When Biospheres Collide" (about the history of NASA’s planetary protection program):

“"We are bathed in Earth organisms, which makes finding our own kind of life palpably easy and detecting indigenous life on other worlds much more difficult. We are not exploring the solar system to discover life that we have brought with us from home, and we are aware that Earth organisms (read: invaders) could very well erase traces of truly extraterrestrial life."

"Likewise, we don't know what would happen if alien organisms were introduced into Earth's biosphere. Would a close relationship (and a benign one) be obvious to all, or will Martian life be so alien as to be unnoticed by both Earth organisms and human defenses? We really have no data to address these questions, and considerate scientists fear conducting these experiments without proper safeguards. After all, this is the only biosphere we currently know - and we do love it!"

The life might be harmless and benign. But if the returned life was so alien that Earth organisms defenses don’t notice it, then it could also be able to overwhelm us, live inside or on us, and our bodies would mount no defenses against it. That was Joshua Lederberg’s insight originally.

So, NASA knew that they had to take precautions, and did try. But with hindsight we can now see that the way they did it was woefully inadequate. Indeed, they also didn’t even keep to the provisions of their own plan, with many failures of procedure at the time, and the reasons for that are instructive also.

NOWADAYS CONSIDERED TO BE MAINLY OF INTEREST AS A VALUABLE LESSON IN THE THINGS THAT CAN GO WRONG

The history of the early precautions against life returned from the Moon are now thought to be of most use as an example of the things that can go wrong, for planning about any future sample return handling facility. See 7 Lessons Learned from the Quarantine of Apollo Lunar Samples

First, the guidelines were only published on the day of the launch of Apollo 11 to the Moon. There was no opportunity for expert peer review of it. This would not be permitted nowadays in our much more legally complex world. Indeed there would be numerous laws to be passed before that one, in a complex process, requiring domestic legislation, several laws, some of which can only be passed after others, international agreement, and even the need probably for domestic legislation to be passed in other countries as well, since the returned sample could impact on them also, even if they are not involved in the mission in any way.

Also, the people who operated the quarantine facilities for the sample return hadn’t had adequate training and experience and tests of their ability to respond and take the right precautions, before they were faced with actually dealing with samples returned from the Moon.

APOLLO 11 SPLASHDOWN - FIRST FAILURE OF THEIR PROCEDURES WHEN THEY OPENED THE COMMAND MODULE DOOR

The biggest issue is what happened after the Apollo 11 splashdown. The mission planners made an “on the spot” decision to ignore one of the key provisions in their plan when Apollo 11 landed in the sea.

As Carl Sagan put it in “Cosmic Connection”:

"The one clear lesson that emerged from our experience in attempting to isolate Apollo-returned lunar samples is that mission controllers are unwilling to risk the certain discomfort of an astronaut - never mind his death - against the remote possibility of a global pandemic. When Apollo 11, the first successful manned lunar- lander, returned to Earth - it was a spaceworthy, but not a very seaworthy, vessel - the agreed-upon quarantine protocol was immediately breached. It was adjudged better to open the Apollo n hatch to the air of the Pacific Ocean and, for all we then knew, expose the Earth to lunar pathogens, than to risk three seasick astronauts. So little concern was paid to quarantine that the aircraft-carrier crane scheduled to lift the command module unopened out of the Pacific was discovered at the last moment to be unsafe. Exit from Apollo 11 was required in the open sea."

The original plan was to lift the module out of the sea in a crane and only open it on deck, enclosed, so making sure that no dust from the Moon could fall into the sea. But they decided the crane was unsafe. Rather than leave the astronauts bobbing on the sea, getting seasick, cooped up in their command module with the world watching, while they tried to figure out a solution - the planners made an on the spot decision to send a helicopter crew and to use divers in a boat to just get them out of the command module as it bobbed about in the open sea. This meant that though they did get into decontamination suits, it was already too late.

Diver opens the door to the command module as it bobs in the open sea and crew get out, already dressed in their decontamination suits. Some of the dust from the Moon that got everywhere inside the module surely got into the sea at this point. This was not part of their plans, but was an “on the spot” decision by mission planners because the crane that was going to take the module out of the sea had malfunctioned, and they felt that it was unacceptable to keep the astronauts waiting while they fixed it.

When they opened the module door, some of the dust that got everywhere inside the module must have got into the sea at that point - you could hardly think of a worse place to contaminate with life from the Moon. For instance suppose there was some photosynthetic life form on the Moon that was capable of out competing the photobionts in our ocean? Then by now the entire ocean biosphere would be transformed. Only those that could eat those life forms, whatever they were, would have survived.

There were other breaches of their protocols later on, due to inexperienced staff at the lunar sample handling facility, also Buzz Aldrin has an amusing story about how ants somehow found their way into the quarantine facility, while they were in it, through some crack or other. But that first one was the biggy. When Carl Sagan saw what happened, he was appalled.

The Apollo 11 procedures are of most interest to planetary protection researches as a valuable lesson that has highlighted issues like this.

From left to right, Neil Armstrong, Michael Collins and Buzz Aldrin in their quarantine unit after return from the Moon. This was largely a symbolic gesture and didn't do much to protect Earth from microbes on the Moon, if there had been any. They had already been taken out of the command module into an open boat in the ocean, and Buzz Aldrin has an amusing anecdote about the time he noticed tiny ants that found their way into their quarantine module. Even if they had carried out the protocols perfectly, they wouldn't have protected the Earth according to present day understanding.

Their quarantine protocols were published on the day of launch with no opportunity for peer review. The duration of the quarantine of three weeks was also arbitrary. Leprosy, for instance, has a latency period of decades.

Also, there was no scientific justification for the length of the quarantine period, for instance Leprosy has a latency period of decades. Even rabies nowadays is thought of as needing a quarantine period of three months. Nor are humans the only creatures that could be impacted - microbes returned from elsewhere could be harmless to us, yet still severely impact our biosphere, or other lifeforms we depend on, or that matter to us. So the quarantine again was largely symbolic. The whole thing was mainly symbolic after that first breach though, showing they cared, but not actually doing much to keep life from the Moon away from Earth’s biosphere.

OTHER WAYS ET MICROBES COULD HARM US

There are many other ways that microbes could harm us, without attacking any Earth life directly. The harm could just be an accidental result of something they do.

I made this “future fake news” story based on one of the examples experts have used, blooms of algae. It’s to illustrate why planetary protection experts are firm in saying that we have to take precautions to ensure something that this can’t happen.

The photo there is a detail from an algal bloom of Lake Eyrie in October 2011 during its worst cyanobacteria bloom for a long time. The cyanobacteria produced microcystins which is a liver toxin and can cause sudden death in cattle within hours, also often kills dogs swimming in the water and is a skin irritant for people.

The algae is not "keyed to the hosts" in any way, and it is no advantage to an algae to kill cattle or dogs. It's used as an example of one way that life from another planet could harm our biosphere

I made this “future fake news” story with this online Newspaper generator

As Chris Chyba put it in his abstract:

"It is unlikely that these cyanobacteria evolved the toxins in response to dairy cows; rather the susceptibility of cattle to these toxins seems simply to be an unfortunate coincidence of a toxin working across a large evolutionary distance"

Though we have a much more detailed picture now, with more in depth understanding of capabilities of extremophiles particularly, and of how small a microbe could be (experts say it could be as small as 50 nm in diameter potentially if not based on DNA), astrobiologists were already saying loud and clear that there is a need to take precautions at the time of Apollo 11.

EVENTUAL PROOF THAT THE SAMPLES WERE HARMLESS

At the time they already thought the risk was tiny, knowing that the Moon had no atmosphere. But Carl Sagan had suggested there could be moisture deep below the surface where life could survive and grow, and that though life actually growing on the surface was impossible, hardy viable spores were not.

So one of the first things they did with the returned samples was to test to see if there were any viable spores in them. There were none, not in the few samples they looked at carefully in this way.

They continued to take these precautions until Apollo 14.

WHAT WILL NASA DO WITH SAMPLES COLLECTED BY CURIOSITY 2020

NASA plan to cache samples from Mars on its surface in the 2020s, and return them in the 2030s. But they haven’t yet said anything about how they plan to deal with back contamination issues during that mission, AFAIK. It’s for the next decade and they only plan one decade in detail at a time.

Artist's impression of a sample return mission from Mars - image credit ESA. It would carry back samples totaling a little over half a kilogram of material, at a cost of many billions of dollars.

With this background, and just the sheer amount of legislation needed to return an unsterilized sample from Mars, especially when you don’t know what is in it, I expect the NASA sample return mission plans to be radically altered when they get the “sticker shock” of the cost of the facility to receive it (half a billion at last estimate some years ago), realize how extensive and complex the legal situation is, and also realize that, as a result of all the lessons learnt from the Apollo precautions, the facility is required to be up and running before they launch the mission to return the sample.

The sample probably won’t have life in it, because they would return it without testing for the presence of life first, just organics, which our Mars meteorites have anyway, believed to be created in abiotic processes or delivered by meteorites to Mars. They also won’t attempt to approach sites that are most likely to have life as Curiosity 2020 won’t be adequately sterilized to do that.

But on the probably small chance that it does have viable spores in it, they would have to take all these elaborate precautions.

I can’t see them doing that for less than a half kilogram of material, not thought likely to have present day life in it. Instead, surely they will sterilize the sample before returning it, in some way. Or I wonder if they might choose to return it to above GEO which would have none of these legal issues, and would save half a billion dollars, or probably more, just on the receiving facility and simplify it in many practical ways and not have the legal issues. They could then send equipment from Earth up to GEO to study it, and return sterilized fragments of the samples back to Earth. If all that is carefully planned and executed I could see it being done under COSPAR within the existing legislation without modification.

For more about all this see my book on planetary protection, OK to Touch Mars? Europa? Enceladus? Or a Tale of Missteps?

This is covered in the section: Planetary protection - researches by Sagan and Lederberg onwards - and Zubrin's arguments and other sections linked to it.