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: After Jean kills Charles, Jean's house (which is suspended in the air) crashes downwards. Watch the scene very closely, and you can see some minor slip-ups in the computer rendering that give it away as a CGI shot. For example, some of the debris on the walkway in front of the house vanishes after landing. A very small piece of debris on the walkway is also seen "popping" upwards for no reason after having sat still for several seconds. Also, right before the shot ends, a few piece of debris in front of the middle window on the roof begin to flop about before flipping upwards in defiance of gravity. There are also several other minor issues with the shot. (Jean is no longer using her powers once the house falls, so this can't explain it).