An argument that is often used by believers in unusual theories is that since some more conventional explanations require that a person has made a mistake, and since that person is a highly trained expert, then that means it's highly unlikely they would make a mistake, and hence the conventional explanation is highly improbable, and so this puts their unusual theory at the top of the list.
The classic example here is with UFOs. The "expert" witness is often a pilot, and ideally a military pilot. If a pilot reports they saw some strange flying craft, then, for some UFO fans, the only possible explanation is that it is, in fact, a strange flying craft. This conclusion is reached because the pilot is "highly trained" and hence it is thought to be impossible that they would misidentify Venus, or Mars, or another plane, or a bird, or a balloon, as a strange flying craft.
And yet experts DO make mistakes. Pilots actually misidentify Venus as an oncoming plane relatively frequently. Pilots land on the wrong runway, or at the wrong airport. Pilots think they are upside down when they are not. Pilots misidentify oil rig lights in the ocean as lights in the sky.
The fallacy extends to other conspiracy domains. With 9/11 we have some "highly trained" engineers who can't immediately (or even eventually) wrap their heads around why World Trade Center Building Seven collapsed in the way it did. With "chemtrails" we have some scientists from various fields who have been convinced by specious arguments about contrails. And of course, there are more mainstream areas where it comes into play. There are anti-vaccine "highly trained" experts. There's the "highly trained" Dr. Judy Mikovits who made the glib but error-riddled "Plandemic" video. There are "experts" who think that really low levels of radio waves can have serious health effects.
Experts make mistakes. I think it might be useful to gather examples of expert mistakes. Not to make the argument that experts are idiots - indeed many of the people involved are highly intelligent, highly trained, capable, and experienced. We should also be cautious to not overstate the prevalence of mistakes. In many cases, they are indeed quite rare. The point here is that mistakes are possible no matter how talented and experienced the expert is. The ultimate point is that one should not discount that possibility, especially when the alternatives (aliens, vast hyper-competent conspiracies, chemtrails, etc. ) are things that are greatly lacking in evidence (and often with significant counter-evidence.)
Such discussions often devolve into intractable subjective assessments of probability: "sure, experts make mistakes, but how likely is it that two experts would make mistakes, or that an expert would make a mistake on the same day that some else odd happened?" These rebuttals are not entirely without merit - often two or more things happening is less likely than one thing happening (unless the events have some causal link.) But any such discussion would really benefit (on all sides) from a deeper understanding of the types of mistakes that experts make, and (where possible) how often they make them.
So I'm starting this thread as a place to gather illustrative examples that will help illuminate the fallacy, and shed some clarity on the "how likely" argument. Let's collect cases where experts got it very wrong.