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: When Wolf Jackson touches a headstone as he and the police are about to raid the church, you can see the headstone wobble when his hand lightly touches it, revealing it to be made of a soft, perhaps Styrofoam-like material and not a real headstone.
Revealing mistake: We see Caesar's Palace in 1966. There are 1970s and 1980s cars in the driveway.
Revealing mistake: During the naval battle, after an underwater shot of a man being eaten by a shark, check the poop deck on the left, and you'll notice a bulky floor with wrinkles around where people fighting and falling bounce on, revealing a tarp hiding a safety mat.
Revealing mistake: During every flashback of the altercation leading to the homicide, the beer bottle completely shatters from just a touch like a sugar glass movie prop. The first time we see the flashback, it is also clear that the minute pieces fall off the table in the first shot, but in the reverse angle, the bottom of a broken bottle is resting on the table.
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 Gary is being interviewed at the end, Slimer and the Dragon fly over them and away. However, Gary keeps staring at the entrance door; seemingly, the cue didn't work here. A shot later, from a different angle, he is suddenly looking ahead.
Revealing mistake: While they drive the truck, searching for the Red One, the truck is in the park position. The gearshift is up, yet the scenery shows them driving.
Revealing mistake: In many shots where Ryan is live streaming his magic show, Aiden's phone screen reveals she is actually recording in the iPhone Camera app. (00:01:17)
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.
Revealing mistake: Real obvious CGI when Alice gets hit with a lamp, exposing her right eye and cheek.
Revealing mistake: When Jessie falls on her back in the breaking into the White House, you can see she is surrounded by complete bullets, perhaps blanks. One would expect the floor to be covered with used bullet cartridges. (01:35:00)
Revealing mistake: In the first scene, they skid around a corner; there are many other skid marks on the same corner from previous takes.
Revealing mistake: At the beginning of the movie, you can see the back of the CB radio, and there is no antenna connected, just power.
Revealing mistake: When Professor von Franz (Willem Dafoe) sets fire to Knock's coffin, the flame jets can be seen under the casket.