Continuity mistake: When Magneto is moving the cars on the bridge at the end, a reddish van in the center of the road is moved to the side. A couple of shots later, the same van is again moved from the center to the side.
Continuity mistake: When Magneto and Charles Xavier are in Jean Grey's house (when she is older and making everything around her float in the air), watch the book shelf when it falls down. At least 25 books fall out of the top shelf and in the next few shots you see that the number of books fallen has minimized greatly.
Revealing mistake: When Bobby and Kitty are skating on the frozen fountain, in one shot Bobby's skates can be seen to be metal covered in ice, rather than the solid ice we saw when he made them.
Revealing mistake: When Magneto is holding Wolverine up in the air in the forest you can plainly see one of the wires used to hold him up.