Revealing mistake: When Mr Bean's book falls apart, he tries putting the book back together, and he holds one page up to the camera and it has a slight reflection as if it is glossy paper. As far as I know old books like the one Mr Bean had don't have pages like that. It's an obviously fake book.
Revealing mistake: When Irma screams at seeing Mr Bean with his jumper pulled over his head, all of the other cinema patrons jump with shock - except for the woman directly behind the two of them, on the right of the screen. She just grins and laughs, forgetting that her character shouldn't be in on the joke.
Revealing mistake: In the train scene when Mr. Bean plugs his ears with his socks you can see that he didn't use the ones he was actually wearing before. Once he took them off he placed them in front of him but reached slightly to his right for the ones he plugged in his ears. Also those where still flat like after ironing them and didn't look worn at all.
Revealing mistake: When Bean silences the guest in room 427 after they were playing loud music, he accidently locks himself out. Later when Bean crawls under a carpet past his room to hide from a kissing couple, room 427 has changed into a fire exit (you can see the room number has been replaced by a fire exit sign).
Revealing mistake: At the end when Mr. Bean blows up the paint and it splatters all over the walls in his apartment, he comes back in and his doorknob is covered in wet white paint which he pushes shut. Only problem is that when he pulls his fingers back, there is no wet paint whatsoever on them. (00:23:30)