i noticed a blooper in ESB during the luke/vader duel in the carbonite chamber. after vader falls of the platform, luke hops down after him. if you watch carefully in the lower left hand corner of the screen, you can see that mark landed on a trampoline and his head bobs back up into the shot. it's pretty funny.
also, in rotj, in the space scene outside the death star immediatly before sidious arrives, some of the TIE fighters disappear the instant just before the scene changes to the death star interior.