With all the questionable changes George Lucas has made to the movies over the years, he's somehow never fixed these. Thanks to Gamma Squad for linking here - they've got a load of great Star Wars stuff to check out. Also Ernie's House of Whoopass, Double Viking and Slashfilm - much appreciated!
When the stormtroopers break into the control room, the stormtrooper on the right of the screen hits his head on the door frame. On the DVD release they've added a thump when he hits it.
When Darth Vader and Ben are fighting, Darth's chestplate is on backwards and the belt "buckle" is as well. It appears that the entire shot was reversed.
When Obi-wan and Luke are watching the hologram of Princess Leia projected onto a small table, the objects on the table change position and number in each shot.
When Leia shoots a hole in the detention area wall for their escape, the hole is not big enough to jump through, but when they all jump in, the hole is much bigger.
When Luke is in Obi-Wan's house trying Anakin's lightsaber, the coat behind him keeps appearing and disappearing.
When one of the stormtroopers unties Han Solo before he is put into the carbonite, watch his wardrobe - he has a disappearing / reappearing jacket on. This is fixed to an extent on the DVD - the jacket is now lightened to almost match his shirt, but it's still shiny like leather.
After Luke says to Yoda, "You want the impossible," Yoda's position jumps from the ground to a rocky boulder, where he then lifts Luke's X-Wing fighter from the swamp.
When the stormtroopers push Leia into the cell in Cloud City, she is wearing a completely different outfit from when she was taken captive.
Viewed from inside, the hotel where Leia, Han and Chewbacca are in Cloud City has a flat glass roof, but viewed from outside, it is flat with sloped edges.
When Han and Leia kiss in the Millennium Falcon, Leia's arms are around Han's head in the wideshot, but not in the close-up.
When Lando says "Go on, you pirate" his entire outfit is reversed - his shoulder holster strap goes the other way, and his rank insignia changes side too.
When Lando is hanging over the pit, he has gloves on, then he has no gloves, then they're back and so on.
When Leia offers the Ewok food, she holds it out in her left hand, but from a different angle it's in her right.
After the heroes get snared and surrounded by Ewoks on Endor, the Ewoks mistake C3P0 for a god and start bowing. There is one Ewok which only has holes where the eyes and mouth are and you can see the eyes and mouth of the person inside the Ewok costume.
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.
Qui-Gon's "comlink" is actually a Gillette Sensor Excel woman's razor.
When Anakin attaches 3PO's right eye, one shot has the eye in his left hand, then the angle changes and the eye is being applied with the right hand.
Obi-Wan's braided hair keeps changing sides, and getting longer and shorter, most noticeably in the scenes around the underwater city.
When the queen is being escorted by some droids after she has been captured, the shadows change completely. When we first see everyone walking past, their shadows stretch ahead of them, but when we see them walking towards Obi-Wan and Qui-Gon, the shadows are really elongated and at an angle.
When Palpatine and Queen Amidala are conversing in his office (when Palpatine refers to Valorum having 'very little real power') his outfit switches from a green breasted cloak with a blue collar area to a green breasted cloak and dark green collar. It switches back after one shot.
During the scene when Obi-Wan attempts to capture Jango Fett as he departs Kamino, Fett and his son Boba can be seen about to load crates onto his ship. As soon as Boba warns Jango about Obi-Wan's presence, Jango tells Boba to get in the ship, the scene switches view, the fight between Jango and Obi-Wan starts and where did the crates go?
When Anakin and Padme are about to eat, Anakin uses the force to levitate her pear-like fruit to his plate. He then proceeds to cut it pretty much in half. However, the piece he sends over to her is much smaller than half - we even see the remainder of the pear on his plate which is substantially larger than it was in the shot before.
The twin-engine speeder which Anakin is using to pursue Zam is equiped with four antennas, two of them between the headrests. In the shot when Anakin is trying to catch the falling Obi-Wan, the two antennas in the middle are missing.
When Anakin and Padme are standing outside the Lars' house, we see their shadows projected on to the side of the house (with Anakin's shadow looking worryingly like Darth Vader's). However, in the next shot their shadows have moved about 4 feet to the left, and the shadows on the ground also change size and intensity between shots several times.
During the sequence where Obi-wan goes to meet Yoda regarding the absence of Kamino from the Jedi Archives, look closely at the collection of children Yoda is training. While most are human, there is one that is alien, with a reptilian face of dark skin. He sure moves fast, because at first he is at the back in the centre of the group of trainees, in the next shot he is at the front of the group of trainees, and by the end of the scene, he is on the far left, yet we never see him, or any of the other children, move.
When Obi-Wan meets Anakin on the landing bay of the volcanic planet, Obi-Wan comes down the ramp from the ship and removes his cloak. It lays across the end of the ramp. The shot changes, then comes back, and the cloak is now bunched off to one side.
During the Anakin/Obi-Wan fight scene Obi Wan switches lightsabers with Anakin. When Anakin is strangling him we can definitely see that Obi-Wan is holding Anakin's lightsaber in his right hand (note the hilt design), but when the shot is reversed Obi-Wan is holding his own lightsaber in that hand.
When Anakin is in the medical centre lying on his back being transformed into Darth Vader, you can see the main part of the mask being lowered into place but Anakin is not wearing the neck section of the mask that it connects to. We then see the main part lowering directly downwards, in the next shot the neck section is in place.