Days of Future Past: Part 2 - S1-E12
Continuity mistake: When Mystique first enters the room where she plans to murder Senator Kelly, there are two bookshelves on the left, with the one closest to the camera being shorter than the one next to it. When Gambit and Mystique are getting ready to square off, the bookshelves are in the background, and they have swapped places. (00:15:52 - 00:16:15)