Revealing mistake: When Elly is "skating" on the black oil while fighting the bad guys, we can see that it is Bryce Dallas Howard's stunt double in a few shots where it briefly shows her face.
Revealing mistake: We see Caesar's Palace in 1966. There are 1970s and 1980s cars in the driveway.
Revealing mistake: When Deadpool dives away from Mr. Paradox and catches his Tempad, he falls towards the employee he has been harassing all along. As the camera closes in to the guy, he is holding a coffee mug, but in the next shot, the mug vanished. This wouldn't be a mistake and is actually a clever hook, since that is the same mug that appears in the intro and the implication is that Deadpool got it at that point in time. However, the implementation seems to be faulty since Deadpool warps away with both arms pointing up, facing away from the guy, who already isn't holding the cup anymore.
Revealing mistake: When Constance's baby is taken out of the water, she is an obvious doll who doesn't wink or move a single inch. She even cries, but her mouth doesn't move. In some shots, she is replaced by a real baby.