Roswell
I suppose all true UFOlogists already know about this, but I either didn't know or had forgotten: under the Freedom of Information Act, the FBI has released a one-page teletype describing the flying disc found near Roswell in PDF format. The teletype is dated July 8, 1947, and says, among other things: "The disc is hexagonal in shape and was suspended from a ballon by cable, which ballon was approximately twenty feet in diameter." Aside from the unintentional amusement value of the phrase "the disc is hexagonal," this teletype would seem to suggest that the "disc" was not nearly big enough to be a flying saucer, and was suspended from a weather balloon. Not much of an alien vessel.
But I'm sure all true Roswellphiles believe that this is a fake teletype, released to distract attention from the real UFO that was found.