Revealing mistake: Obvious stunt man for Carrie Snodgrass due to having brown hair while she's a dark blonde when hanging from an axe handle near the end of the movie. (01:35:09)
Revealing mistake: In the taxi scene Ferris' dad is reading a newspaper. On the front page is an article with the headline "Community rallies around sick youth." As the camera zooms in it's a fake headline to a real news article. The news article begins, "A Chicago policeman killed himself today after he... a Park Ridge office and... workers with a... revolver as he searched for his estranged wife who was hidden beneath a desk by a fellow worker".
Revealing mistake: The chairman parks his car in front of Bel Fuse Ltd. And goes upstairs. His car explodes, but does it in front of a different building, probably switching to a stage building front; the metal doors don't have vents anymore and there are visible join lines in the asphalt and the cement. (00:15:10 - 00:15:40)
Revealing mistake: The scene in the locker room when the bullies are dunking Marty in the toilet. Marty's "body" is a fake prop used - on the naked body mysteriously there is no penis or testicles. In addition to this, despite being turned completely upside down, Marty does not struggle in the slightest as he would in real life - he does not struggle at all exactly because it is a prop body being used.
Revealing mistake: During "Never Say Never" Henri instructs Chantal to fly Fievel to immigration where he should be able to find his family. At one point during this shot, Henri's right wing casts no shadow on the Statue of Liberty - only an outline of the wing's shadow is visible. (00:23:25)
Revealing mistake: When Eddy's mother is lying dead on the bed, she is breathing.
Revealing mistake: When Louise pulls off Joel's trousers, we see that he is wearing briefs under his boxers.
Revealing mistake: During Stimpson's speech toward the end of the film, you can see that it's John Cleese himself giving his sleeve a good pull under the table, when his taped-on sleeve accidentally comes off.