Challenger Disaster
This common lie raised by flerfs maintains that in 1986 the Space Shuttle Challenger did not explode and that the 7 astronauts that were on board were not killed and are still living. The claim is accompanied by a meme that purports to have found images of the dead astronauts living in plain sight in the USA.

One example of a flerf pushing this is Earnest Pearson.
This claim has been comprehensively debunked many times over. One example is this video from Astronomy Live hosted on RedsRhetoric channel. In that video each claimed image is compared to the actual images of the astronauts using face comparison tools and the results clearly show that the claims are baseless.
This is a particularly offensive claim to make since it denies the genuine loss of the Astronauts and it disrespects their sacrifice. MCToon will cancel your debate with him if you go here.