Continuity mistake: When Luke is prodded out to 'walk the plank', into the Sarlaac pit, he jumps off the board and springs himself back up. If you watch this carefully, you can see that only one hand catches the board and that just by the fingertips, but in the next shot both hands are springing him upwards.
Continuity mistake: When Jabba pulls Leia's chain, there is a long shot as she leaps up onto his throne, and he is holding his wine goblet in his left hand, and pulling her chain with his right. However, in the very next shot (in which he says "Soon you will learn to appreciate me"), the entire frame is reversed. This is evident in Jabba's holding the wine in his "right" hand, his higher-placed nostril (which is normally on the left on his face) is now on the "right," Leia's reversal of her armlet, etc.
Revealing mistake: When 3PO and R2 are being assigned to their droidly duties in Jabba's palace, there is a shot of a poor droid having hot irons branding his feet. The irons are never closer than an inch from the droid's feet, and the steam that allegedly comes from the burning is obviously being shot out of holes in the droid's feet.
Continuity mistake: When Vader's shuttle is approaching the Death Star, the crew in the control room of the Death Star are wearing black gloves. One of them then reaches forward to press a button, but in the close-up of his hand pressing the button, the hand is not wearing gloves at all. This can only be seen in the widescreen version.
Continuity mistake: When Lando says, "Fighters coming in," a whole bunch of TIE fighters come towards them. There's initially a lot anyway, but then after about a second, two horizontal lines of 4 fighters appear from nowhere (centre screen, underneath the Super Star Destroyer). They suddenly just pop into existence, large enough that we should have seen them approaching.