Continuity mistake: In the scene where the social worker is at Mr. Hillard's house and his so-called "sister" (Mrs. Doubtfire) is trying to find a face in the kitchen, one moment the wig is all messed up from being caught on the door. In the next scene, where she is looking in the fridge to find the face, her wig is perfectly in place.
Other mistake: When Chris sees Mrs. Doubtfire in the bathroom she is standing up and peeing, in costume. All the scenes where Daniel is putting the costume on shows that the fat suit is a body suit, in one piece, and there's no opening in the front. So he shouldn't have been able to pee without taking off the whole suit.
Stupidity: Mrs. Doubtfire used the bathroom without first locking the door, enabling Chris (or anyone else) to walk in on her. A man who is dressed as a woman and doesn't want anyone to find out would make sure the bathroom door was closed and LOCKED. (01:07:09)