Deliberate mistake: Almost trivia, but not quite. When Mystique's looking through the guard files to find a suitable candidate, lots of photos flash up very quickly before she finds the guy she uses. Of note is that Bryan Singer pops up in guard uniform (with his own name), and so does someone called "Donner, L". This is most likely referring to Lauren Shuler Donner, one of the producers, and sex is specified as "F", but the photo is of a stubble-coated man. (00:23:10)
Continuity mistake: In the final fight, Deadpool says to Cable "did you feel that too?" and there's no-one behind them at all. Cable moves forwards to kill a guy on the ground, and a fraction of a second later, as Deadpool says "only best buddies execute paedophiles together" an orderly has appeared in the background, who gets shot.