Peggy Brandt tells the police officer at Ripley Auto Finishing that she is a reporter from the Evening Star. Later, when introducing herself to Stanley, she says she works for the Tribune (although she could be freelance, or it could be a "journalists tell lies" joke).
Dr Neuman tells Stanely Ipkiss that according to Norse mythology Loki caused so much mischief that Odin banished him. This is incorrect, Loki was chained to a rock while a serpent dripped venom onto his face, his violent shaking is how the Norse explained Earthquakes. This error is corrected in the sequel Son of the mask.
In the final shootout in Niko's club, Dorian's bad guys are shooting at Ipkiss (The Mask) as he hides behind the bar. As they finish shooting the center guy removes and drops his gun's clip. When Ipkiss jumps up and sits on the bar the center guy removes the same clip again before inserting a new one.
When Stanley wakes up to Lt Kellaway banging on his door the mask is in his hand and laying half on the corner of the red pillow, half on the green bed sheet. But in the close up he's in a slightly different position and the mask is now in the middle of the red pillow and you can't see his hand at all; then it swaps back again.
When the police are searching for The Mask in the park, there's a shot of a number of things on the ground that they have already found. We see at least five or six items there that the two cops later list as they pull out of his pockets: a fish, a mousetrap, a bike horn, a squeeze toy, and a rubber chicken.
Part of The Mask's clothing is shot off, and reverts to being a piece of Stanley's pajamas - but when the police frisk him in the park, they produce all manner of objects, (such as a bazooka from his pocket), which do not revert.
When Milo the dog is holding the mask and a guy is pulling his back legs, the floor beneath them is a blue and white pattern, when the mask is attaching to Milo's face the blue disappears and the floor is all white.
When Stanley (as The Mask) eats the bomb, the red LED countdown timer has become a black sticker with white numbers, which reads "0100". This number makes no sense in the context of the scene, as the previous time shown (on the red LED countdown timer) read "0:06".
The bomb at the nightclub has two dials on the top. One is a red LED digital countdown timer, the other is a tumbler-style counter behind a raised clear plastic cover. When Stanley (as The Mask) eats the bomb, the tumbler-style counter has become a black sticker with white numbers.
After Stanley (as The Mask) uses the Squeeze Me Gently horn and shatters the car windows, (as he walks away) the car no longer has any glass derbies on the hood, nor does it appear to have any recently broken windows.
After Stanley throws the mask out the window of his apartment the device used to throw the mask back into the apartment is clearly visible moving upwards towards the top of the window frame as the mask flies back through the window.
Since the mask is not aerodynamic, when thrown its velocity has two components: a horizontal one being slowed down by air resistance, and a vertical one where it falls down at the same rate as if it had just been dropped. However, men jump up to catch it, and fall down again much faster.
The Mask makes the words 'Rip Off' out of 'Ripley Auto Finishing,' which only has one 'F' in it. However, it is shown throughout the film that the Mask has the ability to make things materialize out of thin air.
The detective says that the bank was robbed by a guy in a green mask, but the surveillance tape is black and white. He could have connected this crime to the events of the previous night, where there were eyewitnesses.
In the first scene, when the diver is trying to break the chain on the chest containing the mask, the pipe that falls from above makes a clanging sound as if it were metal but it is in fact PVC pipe. It appears to be painted black (PVC water pipe is usually white). The ridge on the bell end of the pipe has a rubber gasket in it so the pipe will seal when connected to another piece of pipe.
When Stanley first arrives at his apartment, there is a 5 1/4" floppy disk in a sleeve setting on the keyboard of his computer - as if it was one he used regularly. However, the computer only has a 3 1/2" floppy drive, not a 5 1/4".