Stupidity: It's stated that Imhotep will fear cats until he has fully regenerated and two different scenes show him fleeing in terror at the sight of a cat. Despite this, none of the characters that Imhotep is trying to kill that are fully aware of his weakness even think to have a cat with them at all times.
Stupidity: Al is supposed to be a super serious toy collector and seller, but he handles valuable toys without cleaning the cheese puff dust off his hands.
Suggested correction: It's also established someone is coming over to clean said toys, dust included, and when he does the handling, he's just been sharply awakened by the TV therefore is off-kilter.
Stupidity: It is pretty silly for Martin Lawrence to go undercover as a police officer when it would make way more sense to go undercover as a janitor/custodian since his main goal is to gain access to the vent. A janitor would be a way less conspicuous disguise than a detective and slipping through a backdoor entrance disguised as a janitor should not be all that complicated for someone as savvy as Myles. But of course this would make a far less entertaining film.
Stupidity: After leaving the movie theatre, Billy and Jenny have already reached their parked car, opened the car door, and are about to get in when they are confronted by her ex-boyfriend. The next moment she says, "Just walk away," and she and Billy do just that, ostensibly to ignore and avoid the ex-boyfriend. Why didn't they just finish getting into the car and drive off? Where are they walking? It makes no sense.