Continuity mistake: When Ed is skating through the park and the baby gets mixed up with the players, they throw the baby through the hoop. If you watch, the man who threw the baby through the hoop doesn't have sleeves, but when it shows the baby being caught, the catcher has sleeves. Then it goes back to the person who supposedly caught him, but he has no sleeves again.
Continuity mistake: In the scene near the end of the movie, where Ed and Dexter are about to sneak into Mondo Burger to steal the chemical, when they hide behind the moving Frito-Lay truck they are still in the uniforms from Demented Hills, but in the next shot when they sneak around the corner, they are back in their Good Burger uniforms. And you never see the Demented Hills clothes again.
Other mistake: When Kurt is putting the shark poison in the sauce look at the color of the poison, it keeps changing colors.
Revealing mistake: When Ed and Dexter are in the ventilation shaft, watch when they fall in the trolley thing. If you look carefully you can see Ed's wig hanging on the edge. It's got a good burger hat on it.
Deliberate mistake: As the Mondo Burger building is falling apart, customers and employees run out the doors. The glass doors then shatter. Not only do they break at the exact same time, there was nothing to make the windows shatter as hard as they did.