According to one biography, Robin Williams decided to test out the believability of his Mrs. Doubtfire character during filming by going,
as Mrs. Doubtfire, into an adult bookstore and making a purchase. He was able to do so without being recognized.

During the scene when Mrs. Sellner comes to inspect Daniel's apartment and Daniel/Mrs. Doubtfire is serving her tea, the icing on his/her face is melting off.
This was not intentional. The heat from the set lights melted the icing on his face and Robin Williams improvised the bulk of that scene.

The producers fired the original screenwriter because of an "unhappy" ending, where the parents do not get back together.
After several re-writes, with a different, "happy" ending, they realized that the original ending was, in fact, better.
They re-hired the original writer and went back to the original ending.