(at around 14 mins) When Willy has his arms around the children, you can see the top of his walking stick on the right. In the next shot, it's on the left.
(at around 1h 6 mins) When Peter is with Mystique, they zoom in on her face to show her yellow eyes. After Peter's second request, her eyes are blue again for the next two zooms. After they show Peter drooling, Mystique's eyes are yellow again.
When Nacho sneezes on Edward with the chips in his hand, the chip goo is all on Edward. In the next shot of Edward, all of the goo is gone.
(At around 1h 6 mins) When Mystique and Peter in bed, Mystique gains a mono brow at Peter's request. The mono brow stays as she transforms into an incredibly fat and flabby woman per Peter's request, shown clearly when it zooms in on her face. However, once Mystique goes down to start to make out and get it on with Peter, she does not have a mono brow, but only little dots on her forehead.
(at around 1h 10 mins) When the White Bitch is playing "Ms. New Booty" on the battlefield, she has a giant stereo that there was no sign of mere seconds earlier. This is probably a gag.
(at around 19 mins) When Lucy went into the wardrobe, a large number of objects poured on top of her when she opened it. However, when Edward goes in (at around 28 mins), there is nothing in the room and the wardrobe is neat and closed again. This is probably deliberate.
(at around 3 mins) When Silas attacks Lucy in the Louvre, his robe reveals his right leg, and it is dark instead of white (Silas' whole body is supposed to be albino).
(at around 25 mins) When Mr. Tumnus shows his crib, he says, "Look! My Plasma!", but the TV is clearly a Sharp, which only has LCD models.