Continuity mistake: At the end when the children learn that their dad is their new baby sitter, look at Chris' tassels on his coat from when they get to door to when Daniel says 'Go get your stuff' and he runs up stairs - between shots, his tassels are both hanging down, then hanging to the right, then hanging down etc. indicating this scene was shot at least twice.
Trivia: When Stu calls Daniel a loser at the swimming pool, Mrs. Doubtfire then picks up a lime and throws it at Stu, hitting him on the back of his head. Robin Williams managed to hit Pierce Brosnan on the third take and the lime you see was made of foam rubber. Mrs. Doubtfire then tells Stu it was a 'run-by fruiting' - this line was improvised by Williams.
Mrs. Doubtfire: I hope you don't mind me being a tad rude, but... How was he? You know, on a scale of 1 to 10?
Miranda: Well, that part was always... Okay.
Mrs. Doubtfire: Just okay? Well, he was probably a Casanova compared to poor old Winston.
Miranda: What was the matter with Winston?
Mrs. Doubtfire: Oh dear, Winston's idea of foreplay was "Effie, brace yourself."