Revealing mistake: When the Falcon leaves Hoth and is being chased by the Empire, there is a scene where the ship gets rocked about. During the 'impact', C-3PO's arm becomes dislodged, and you can clearly see human skin underneath.
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: When Leia and the crew return to cloud city to rescue Luke from the scaffolding, watch very closely as Lando ascends up the lift, just after the top hatch opens from the overhead angle, you will see that Lando has been replaced with a G.I. Joe look-alike figurine.
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: After C-3PO gets shot, as Chewbacca walks over to the door, his right foot crushes one of the steps on a small set of stairs.
Revealing mistake: After Chewbacca pulls R2-D2 away from the power socket, he runs away, and you can see white rubber soles under his feet.
Revealing mistake: After the headless AT-AT begins to fall over, watch its back leg on the right side of the screen. You can see a broomstick being pushed up from under the set. It raises that foot up in order to ensure that the AT-AT falls to the left side of the screen.
Revealing mistake: In the first shot of Luke riding on Hoth, the tauntaun leaves no tracks in the snow.
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: 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.
Revealing mistake: If you closely when Vader cuts through the three poles at the end of the lightsabre duel, you can see that the sabre never actually goes through the poles. Play it in slow motion for confirmation.