Continuity mistake: Colossus has a dent in his head which is sometimes there and in other shots has disappeared.
Continuity mistake: When Deadpool lies back on the barrels of kerosene, the opening for the drum under his head is shown covered by his back, while the drum under his right arm has its opening near his elbow. In the next shot, the drums have mysteriously turned, with an opening now appearing next to his head, and the one that had been under his elbow, now rotated by 180° near his shoulder. He then flicks a lit cigarette into the air, which is shown to drop into the drum next to his shoulder - except that the drum under his head is the one that explodes first. (00:01:55 - 02:09:00)
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.
Continuity mistake: When Deadpool and Cable confront Russell (Firefist) near the end, Cable glances to his right and says: "There's one bullet left in that gun!" We see a closeup of the semi-automatic several feet away, and the handgun is elevated above ground level, resting on a slab of concrete with a bit of flame burning just inches away. Moments later, as Deadpool pleads with Russell, Cable again glances at the gun, which is now lying in the dirt, below the level of the nearest concrete slab.
Continuity mistake: When Russell and Wade are shown into their cell, Wade makes a wisecrack, gets tased and falls to the floor writhing in pain. Russell is looking down at him and then turns to look out of the cell. When the camera cuts to the wide shot, Wade is instantly on his cot, lying quietly on his back with his arms folded. (00:34:50)