X-Men (2000)
When Wolverine's car hits the tree, front window gets smashed out of the car. In subsequent shots, the window is only damaged and is back in its place.
The star on Magneto's father's coat at the beginning changes sides from the left to the right.
The camera is visible in Cyclops' glasses when he is in Logan's room and says, "Oh, and Logan: stay away from my girl."
Cyclops' gloves are suddenly missing when he's trying to pull Toad's slime off Jean's face.
In the train station, as Sabretooth is coming up behind Storm at the ticket window, he throws the guy behind her out of the way. The line attached to the stuntman is visible in this shot as he is yanked out of the way.
X2
When the X-men jet is trying to dodge the rockets, there are a few close-ups of Bobby showing that the seat in front of him is empty. In other shots before and after the close-ups of Bobby, it shows that Rogue is sitting in that "empty" seat.
When all the X-Men and Xavier are talking with the president....supposedly everyone who is not an X-Men is frozen or time has stood still but in one shot you can clearly see someone blink a couple of times.
Towards the end of the scene where the mansion is invaded by Stryker, Bobby, Rogue, Pyro and Wolverine are running to the secret passage. As they run down a corridor, a camera is clearly visible moving into the roof of the set.
When Mystique injects iron into Magneto's passed out guard, folds in her body costume appear as her abdomen twists.
When Logan encounters Jean in the lab he gets knocked unconscious without his jacket on. Later, when Xavier and Storm discover him lying on the ground, he's wearing his jacket again