Revealing mistake: When The Mask is dancing with Tina and he turns her into a human tornado while spinning her around, none of the onlookers in the crowd show the slightest bit of amazement at the impossible feat that just occurred in front of them, indicating that the extras were not directed to react to the special effects.
Revealing mistake: When Kellaway shows up at Stanley's apartment to question him about his pajamas and Stanley scrambles to stuff all the stolen bank money into his closet, you can see that a few of the bills are blank on the back side, revealing that the money is a fake movie prop.
Revealing mistake: When Milo is trying to jump up to the jail cell, the brick wall is not brick but plastic as can be heard from his paws scratching on it. (01:11:35)
Revealing mistake: When The Mask dances with Tina for the first time, after he swings her around the floor you can see the stunt double's face, older than Cameron Diaz.
Revealing mistake: In the alley, The Mask tells one of the street thugs that he's going to pull his underwear over his head. Here you can see that The Mask's rubber 'skin' is peeling up on the back of his neck.
Revealing mistake: When the mask blows the small horn at the car, just before the windows blow out, you see the dummy in the car. (00:20:08)
Revealing mistake: They put the explosive down near the end and open it with a crowbar. As this is happening, the shot pushes in a tad. Notice a rough cut here as if the box wouldn't open as intended, so the editors spliced the scene together.
Revealing mistake: The first night at the Coco Bongo, when The Mask ducks the goons' bullets, the bullets don't shatter a single corner of the stage behind.
Revealing mistake: When Dorian breaks down the doors of the Coco Bongo, the doors have no hinges to help for the stunt.
Revealing mistake: When Ipkiss is in jail and he is summoning Milo, the dog jumps up through the bars. When Ipkiss pulls him through, one of the bars swings a little bit. It must not have been a very secure cell. (01:13:00)
Revealing mistake: When the super-stretch limo is pulling up to the club, we see window after window go by, then the picture cuts for a moment to the crowd watching. Upon returning to the limo shot, the edge of the front door can be seen on the left side of the screen, ruining the illusion of the ridiculously long limo. (00:36:50)
Revealing mistake: Dorian and his men are watching what we're assuming is a live feed from Tina at the bank. At one point the "video" stops as you can see a guard freeze. From that point on the video is a still picture.
Suggested correction: If you look at the top of the screen showing the bank video, you can see a multicolored distortion. This shows that they paused the video feed.
We don't see them pause the video.
We also don't see their hands, so one of them easily could have paused it.
