Continuity mistake: In the library scene Indy discovers the "X" high up on the balcony. The X is green with a grey background. When he breaks the tile to find the tomb the X has become a faint outline on the floor. (00:27:40 - 00:28:45)
Suggested correction: You still can see one "leg" of the X on the floor, it's only darker than viewed from above because the camera angle and illumination set used.
I think it is meant to be an optical illusion.
The "X" is first shown as a dark green "X" on a beige background. Next, we are shown the same dark green "X" that is barely visible over a green background. I think we are meant to understand that the beige square tiles were lifted away in a cut scene.
I see no reason why they would replace the floor just for the higher shot, it's the same floor throughout the scene. When they enter it's the same floor we see later as they are going into the hole. It's probably not a real marble floor, so they can use a styrofoam or plywood tile that Harrison can lift, one that matches the surrounding tiles. They don't shine as much as the rest of the floor. In the shot up high there is different lighting, so that could explain it. It just appears to be different. Of course, sudden different light can be seen as a revealing mistake.
Suggested correction: Not a mistake, just a different viewing angle.
Continuity mistake: Notice Marty's hair in the scene when he's playing "Johnny B. Goode." It's quite larger than when he says, some seconds later, "I guess that you're not ready for it yet." That's because when he says that, it's a recycled shot from the original BTTF, whereas most shots showing he's playing the guitar are newly made for BTTF2. However, Marty's hair is considerably longer in the new shots. (01:27:19)
Continuity mistake: Early in the film Bud throws his wedding ring down the latrine and then retrieves it with his right hand so it is covered with the blue disinfectant. In a (much) later scene, Bud has a blue right hand, despite not having had one in the intervening period. (01:44:48)
Continuity mistake: In the scene on the plane, you see Bond hanging at the door on the right side of the plane. In the next shot, you see the plane making a left turn. In the following shot, Bond is still hanging on the right-side door. Shouldn't he have fallen into the plane?
Continuity mistake: When Vigo throws the streams from the proton packs back onto the Ghostbusters, there is a wide shot of Egon getting thrown way over to the right. However, when they do a closer shot, he is way over to the left, closer to the other three Ghostbusters. (01:33:50)
Continuity mistake: At the patch ceremony, Chica goes over to Phyllis to get some food and she has her patches on. The patches weren't given out until the next scene.
Continuity mistake: Bernie's facial expression changes from the smile on his face when he first get injected with drugs to a smirk throughout the movie, to no expresssion and back again.
Continuity mistake: When Amy is in the kitchen cleaning, she starts mopping the floor, then dancing with the mop held upside down. The mop starts out wet, but when she's dancing it's dry (either that or she would be getting dirty mop water in her face while the mop is being flung around).
Continuity mistake: At one point, when they time-travel away from "1,000,000 B.C.", Bill is on the left inside the phone booth and Ted is on the right. When the camera switches away, then back, they've changed sides.
Continuity mistake: When Wallace races to get crackers, check the packet's logo. When the camera angle changes it's different. (00:08:57)
Continuity mistake: When Brian spends his first night in the underground monster world he goes in wearing a shirt and boxer shorts. But a later scene shows him wearing a shirt, pants, and shoes. He then goes back home and is back in his shirt and boxer shorts.
Suggested correction: It's entirely possible that he was able to borrow a pair of pants while down there.
I just watched the scene where Maurice and Brian pull the pranks to get the kids into trouble and he is in his pyjamas. Later in the monster world he is in a full outfit including a shirt, pants, and shoes. Then he goes back home and he is in his pyjamas again.
Continuity mistake: When Sam is swinging the shovel at Putnam's car he busts the driver's side headlight twice.
Continuity mistake: When Banner changes into Hulk in the subway station, his beard is gone. When Banner changes back after his "Hulk-out" his beard is back. (00:16:00 - 00:18:00)
Continuity mistake: When Fuller falls to his death, the mannequin used as his stand-in gets decapitated on its way down. It even makes a loud noise. When Fuller lands, however, his head is still on his shoulders.