Because of the position Han assumes when firing his laser in the detention cell corridor, his neck tends to rub up against his armor. Watch his breastplate throughout this scene. Harrison Ford’s make-up rubs off all over it.
The footage of Leia diving into the garbage chute is sped up. Look at the smoke in the scene (and Chewie's legs in the background, in letterboxed copies.).
As Luke gets pulled under the muck, it cuts to a wide shot. As Han yells out "Kid!" and Leia yells out "Luke!" Leia’s cry is mouthed a whole beat early than the actual sound. She may be cut off in the cropped versions. Likewise, when Luke emerges and Leia yells "grab him!" the lip synch is way off.
Okay, EVERYONE knows about this one. As the stormtroopers enter the control room, after blowing the door open, watch the one on the right. As they enter, between the lines "take over" and "see to him" this guy bangs his head on the door.
Once Threepio and Artoo set up shop in the hangar, and Artoo plugs into the computer again, there’s a neat throw-away prop. A clear plastic cube lights up, and slowly is pushed out of the console.