Revealing mistake: When Leia and Lando are holding off the stormtroopers while Chewbacca and the droids escape into the Millennium Falcon, a stormtrooper comes running through the doorway. You can see the burn mark on his armour before Lando shoots him. When the trooper is shot, another trooper comes running through the door, also with a burn mark on his armour. Then Leia shoots another (or perhaps the same) trooper, and the burn mark is once again there before the trooper is hit (watch in slow motion).
Revealing mistake: Right before Luke nicks Vader in the shoulder with his lightsabre, Vader cuts through a railing. There's a hole in the railing exactly where Vader apparently cuts it. Also, if you play it in slow motion, Vader's lightsabre never comes into contact with the railing, yet it sparks.
Revealing mistake: In the original (not Special Edition): When the AT-ATs are attacking the Hoth base, they do a camera angle from inside Luke's cockpit. As his snowspeeder is passing through the legs of an AT-AT, part of the cockpit becomes translucent and you can slightly see the AT-AT through a metal bar.
Revealing mistake: Right after Luke and Darth fight for the first time, Darth gets kicked off of the Carbonite freezer. Then Luke jumps down to fight him again. After he jumps, you hear what sounds suspiciously like a trampoline. Then in the lower right screen you can see Mark Hamill's head pop back up into frame. Widescreen only, although the sound is audible on the 4:3 version too.
Revealing mistake: Look carefully at the end of the cavern where the Millenium Falcon enters the giant asteroid. It's a dark blue circle, because the light is dim. But there is a little white circle on the right and the two circles are much brighter on the left side. It's because it's an image of a planet with a moon.
Add timeDr Wilson
Revealing mistake: When the stormtroopers are firing at Chewie and the droids and C3PO is yelling at R2D2 about the Millennium Falcon's hyper-drive, there is a shot of Chewie being shot at and the beam putting a hole in the wall. In one shot, it doesn't look much bigger than a pinpoint, but in the next shot, the team at Industrial Lights and Magic were kind enough to add a CGI burn mark, which contracts, almost disappears, and then warps as smoke passes in front of it, wrecking the illusion.