For me, flat Earth. The sheer skill necessary to bribe or convince so many governments to gaslight their population, the incredible optics and geodesics work necessary to fake a curved horizon in a plane window and have consistent flight durations, and the maintenance of shadow power across millenia, paints the picture of a group of people way beyond what human competence can achieve.

Of course, there are simpler examples, like John Titor, or Pierre de Fermat’s last theorem proof that he would have done all in his head.

The fake moon landing conspiracy is a good example. Then number of people involved would be pretty big, and the science is complicated. The logistics behind such a big secret operation requires such skill that it is hard to believe anyone would be able to pull it off. If the moon landing was truly fake surely they would have messed up somewhere.

On the other hand many things went wrong in the actual moon landing. There were liftoffs that failed, experiments that went nowhere, targets that were missed, and even people that died. The actual moon landing was a hard task they succeeded at while making a ton of mistakes in the process, just like humans do in the real world when we are at our best.

The "massive rocket" is also an issue: https://www.youtube.com/watch?v=sE-tpiAiiHo

Okay, I agree that the fake moon landing conspiracy would have involved an excessive amount of competence to believe.

That seems pretty different from, "If your entire plan is to back out of a contract, add a clause to the contract that lets you back out of it."

Indeed it is different. My point was simply that incompetence is not evidence against conspiracy. My sniff test was simply emphasizing this point. Further, a sniff test is—by definition—not supposed to be your only marker in evaluating a theory, it is merely supposed to be a marker determining if evaluating further evidence is even worth your time.

The conspiracy theory that Musk is using this twitter scheme to manipulate the markets in his favor does not fail any sniff tests that I’m aware of.

It's a pretty universal reason to dismiss or at least cast serious doubt on conspiracy theories. A close cousin of Ockham's Razor and Hanlon's Law

The amount of competence required to prepare a controlled demolition of a 110 storey trade center which is occupied 24/7 and full of cameras without anyone noticing and associating this prep work with the much publicised plane crashes that happened afterwards would be staggering, even before considering the insanity of such a scheme.

At the risk of opening a can of worms, the best evidence against the considerably more plausible "lab-leak" COVID theory is that manufacturing an alternative chain of evidence that convinces most unconnected foreign experts of an alternative theory (which still points at Wuhan and Chinese market regulations) when they have plenty of reason to find fault with it requires a lot of competence, as well as totally the opposite approach to China's usual way of suppressing stories.

Not GP, but chemtrails and flat earth are obvious ones.