Days of Future Past: Part 2 - S1-E12
Continuity mistake: When Blob is looking out the hotel window, the window panes have been opened at a 90° angle. When the shot changes, the opened panes are no longer visible. When the shot changes again, the windows are slid open, which would make it impossible for the panes to have been positioned at a 90° angle in the first place. (00:09:07)