When Stanley comes home after being in the river, wet up to his belt, he enters his apartment and sits on the couch, and his trousers are dry and clean.
Stanley's suit jacket disappears in prison as he goes to the window to see the dog.
When Tina punches one of Dorian's henchmen, she leaves a scratch mark on his left cheek. After she escapes and runs down the alley way and is stopped by Dorian, the henchman catches up, and the scratch mark has vanished.
Stanley's tie suddenly flips over his left shoulder during the bank account opening scene.
The "loaner" car Stanley picks up at the mechanics is a 1951 Studebaker Champion. However, the car that falls apart on the bridge is a 1950 Studebaker Commander.
The rollers in the printing press all rotate in the same direction, instead of against each other.
When Dorian as the mask sucks the bullets fired at him by Niko into his mouth, they still have the shell casings attached. Fired bullets would not have the shell casings still attached.
The "Cuban Pete" dance is more of a mambo or samba, not a rumba (think "The Girl from Ipanema").
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.
Lt. Kellaway informs Stanley that his apartment manager, "unloaded a couple of rounds of 20 aught buckshot five feet from your door." There is no such shotgun load.
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.
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 first night Stanley sees Tina getting out of the car to enter the club, Tina is wearing a black and white striped outfit. The next morning, Stanley is shown Tina's picture in the paper. In the photo she is wearing the outfit she wears when she sings at the club again the next night, not the striped outfit she was seen wearing when getting out of the car. But it's also possible she changed wardrobe and wears the same outfit night after night.
When Stanley opens the closet to hide the mask, and all the money comes out, some of the bills are clearly blank pieces of paper.
In the scene when Stanley talks to Milo from the jail-cell window, you can see closely when he lets go of the bars they bend and flex a little.
When The Mask jumps through the apartment building window, the window's glass surrounds him as he is falling, yet no glass lands around him.
Just before the car windows are blown out by the "Squeeze me gently" horn, you can clearly see a dummy in the car.
Obvious double when Tina is dancing with Stanley.
When Tina is singing "Ain't I Good For You" in the club, near the end, her voice says "honey" but her mouth says "baby".
When Stanley is dancing with the parking meter (when he's sticking his butt in the air) his mouth doesn't match with the words being sung.
When Dorian has the mask and attacks Niko's casino, he tells his men to hold fire. Then he tells Niko to shoot him. When Niko shoots, the gunshot's sound and the gun's flash don't match.
When Niko hits his golf ball (before Dorian arrives) the display says that he has hit the ball 160 yards, but the audio for the machine says "120 yards."
When Milo the dog has the mask and a guy with long blonde hair is pulling his back legs, in two brief shots it's another guy with short black hair.
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.
Dr Neuman tells Stanley 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.
When Stanley recognizes Peggy, he refers to her as the writer of a column entitled "Ask Peggy." Seconds later, she refers to it as "Dear Peggy."
Peggy Brandt tells the police officer at Ripley Auto Finishing that she is a reporter from the Evening Star, but the reporter badge she's wearing says "Edge City Tribune" and she later tells Stanley she's reporting for the Tribune.
When the lieutenant and his men see Stanley running down the alley they unload about a hundred bullets on him, while he's standing right next to a clearly visible civilian. This is not police procedure especially as Stanley was not presenting a clear threat to anyone and was unarmed. Even in a crime riddled city in 1994 its unlikely they would shoot.
The sleeping cop guarding Stanley in his cell has left his break-front clam shell holster un-snapped, allowing anyone to grab his weapon. If he'd left it properly secured, Stanley wouldn't have been able to disarm him.