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 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.