Continuity mistake: In the scene near the end where Ajax has a knife in the side of Deadpool's head and is punching him, later Deadpool pulls the knife out. In a follow up shot there is no hole in the mask where the knife was, but in a later shot the hole is there.
Factual error: As the X-Men land in Cairo, scenes reveal the destruction Magneto is wreaking around the world, where it is simultaneously daylight in New York, Sydney and Cairo - it is possible for it to be daylight in two of these locations at once, but not all three. (01:46:00)