Other mistake: It was established in the first Mask that the mask does not work in the daytime, but it works during the day in this movie.
Continuity mistake: During the scene where the baby is peeing on Tim, in one shot his hair is wet, in the next it's dry and in the next shot it goes back to wet again.
Continuity mistake: When Loki arrives at the Avery's home to check out Alvey, his car skids to a stop pinning a squeeze toy under the front right tire. When he leaves following Tim and Alvey into the city, the squeeze toy is not under the tire. (00:53:00 - 00:56:35)
Audio problem: When Tim and Alvey are chased down the alley by Loki, Loki pulls out a huge fire ball, and then Odin the great god takes over Tim. After Odin the great god says, "I've had it up to here with you!", you can see Alvey in Tim's arms crying, but you can't hear Alvey crying.
Continuity mistake: When Otis (the dog) is in the trap, in the shot before he falls in the piano, Tim is standing in the room, but when the piano closes, he is not there. He suddenly appears in the next shot, so he couldn't have run out of the room and come back in.
Other mistake: When Tim wakes up to feed the baby at night, he smashes the light, but it somehow stays on.
Continuity mistake: When Avery is about to place the kid on the floor to prove his neighbour how he misbehaves, the kid's hair swaps from brushed to messed for a split second, then back to brushed.
Continuity mistake: When the baby starts jumping around the room, a blanket on the sofa disappears between shots.
Continuity mistake: When the dog fantasizes with putting the baby inside a cannon, he puts 6 rockets in his butt. A shot later there's just 5.
Revealing mistake: When the baby fires the big jet of pee it comes out of somewhere next to his knee instead of his groin. Obviously where some sort of hose was placed.
Visible crew/equipment: When the dog hands the baby a rattler, in the close-up you can plainly see it's a crew member's hand in a gray glove which is definitely not a paw.
Continuity mistake: At the museum, when Loki transforms into a beast, a blonde woman grabbing a purse runs by the showcase twice.
Revealing mistake: When Loki disguised as the delivery person grabs the baby, he's replaced by an obvious doll, with pale yellowish skin and lighter hair colour.
Continuity mistake: When the baby pees Tim he is holding a mug with his left hand, but a shot later he is holding a green bottle. Also, while holding the mug the handle changes positions between shots.
Continuity mistake: When the dog crashes against the postman's truck, the road gets covered with thousands of envelopes. Ten seconds later, the road is spotless.
Audio problem: When Tim plays Mario Kart Super Circuit the sound effects are from an entirely different game.
Answer: Otis had been feeling neglected because Tim hasn't had as much time for him ever since Alvey was born, so he attempted to take Alvey out of the picture so he could have Tim all for himself.
Casual Person