I'll start.
Did you know that Wil "Wesley Crusher" Wheaton's first credited acting role was Martin on the Secret of NIMH animated film? That was probably the last time he sounded assertive and tough in an acting role too, and he was both a little kid and a mouse. :rat-salute:
Bill Paxton has been killed by an Alien (as a marine in aliens) by a Predator (a cop in Predator 2) and a Terminator (punk in police station in Terminator)
James Stewart had to take over and direct a movie he acted in because the alcoholic director was so sauced he couldn't do his job.
The mall in the original Dawn of the Dead has no elevator. All the elevator scenes were shot at a different location.
In the finale of the first Macross movie one of the missiles launched as part of a big Salvo is actually a beer can of the animators favorite beer. The logo flashes into view as the barrage roars by.