Continuity mistake: Near the start when Wade and Vanessa are discussing families, in most shots facing her she's got her thumb lying along his jawline, but in most shots facing him it's sticking up by his ear.
Continuity mistake: Deadpool turns all but the last dial on his stove to the max setting in one shot, but in the next they are all turned different directions and the last one is turned as well.
Continuity mistake: Wade makes a red circle around Cable on the cartoon map which disappears between shots.
Continuity mistake: While Wade is shooting the Japanese guy, the sword on his back disappears and reappears between shots.
Continuity mistake: Colossus has a dent in his head which is sometimes there and in other shots has disappeared.
Continuity mistake: The tread on Deadpool's boots when on top of the crashed lorry (just after Domino lands on the inflatable panda) are different from when Juggernaut rips him in half and his legs are behind him on the floor. (01:10:45 - 01:12:35)
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: While Wade fights two guys at his apartment, he uses a pan to stop a punch with the two hands, but in the next shot he uses only one hand.
Continuity mistake: At Hong Kong while Wade is being shot at, he jumps to the bar and leaves a blood stain on it which changes amount and position.
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 it starts to go slow motion and Cable begins to run for the gun you can see Deadpool in the corner running before the next shot where he turns standing, then begins to run. (01:48:31)
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)
Answer: His change of heart came from Deadpool's sacrifice. In the second timeline, Cable saves Deadpool, but Deadpool had no way of knowing. Firefist still has a change of heart because Deadpool was willing to sacrifice himself, even though he was ultimately saved by someone else.
Now, I can accept that in theory, except that Russell repeatedly saw Deadpool putting his ass on the line to rescue Russell. I mean, Russell knew from the very beginning that Deadpool could have killed him (but chose not to) and took some severe ass-beatings on Russell's behalf. Russell was really, really hard-boiled, and I'm not seeing that Deadpool almost getting killed as enough impetus to change Russell's heart. It seems (to me, anyway) it was Deadpool's actual death that changed Russell, such that a mere deflected bullet would not have the same effect.
Charles Austin Miller
Deadpool often mentioned "lazy writing" and Russell having such a change of heart might be an example of it.
Erik M.
Deadpool saving Russell in the film is what made Russell think that they were friends. When Deadpool tells Russell that they aren't friends, he remains hostile toward Deadpool, not believing him when he later admits to caring for Russell. At this point Russell is too far gone and will kill. However, it's only when Deadpool takes a bullet for Russell, fully intending to die in both timelines, that Russell sees that Deadpool really does care about him, and would have died to save him.