Creative Differences: By all accounts it was a smooth shoot, so much so that new scenes were added and filmed just because the actors were having so much fun. However, Mel Brooks and Gene Wilder frequently butted heads, especially over the now-famous "Puttin' On The Ritz" scene which Wilder had to fight for until he was blue in the face just to prove how badly he wanted it keep it. One particularly brutal day ended with the two parting ways after a huge fight, only for Mel to call him that night and apologize in his own special way by asking "Who was that asshole you were fighting with today? If you ask me, he should be fired!"
Deleted Role: Leon Askin played a lawyer (reading the last will) but was cut out.
Igor's hump, which changes position from scene to scene; Marty Feldman decided to do this on his own, without telling anybody beforehand. When someone finally noticed, they added a bit where Frederick does as well.
During a search in the forest, Inspector Kemp's wooden arm appears to have switched sides. On closer inspection, it's actually the entire scene that's obviously been flipped as his eyepatch and monocle are on the other eye, and his badge is on the other side.
Mel Brooks ad-libbed the sound of a cat getting hit with one of Frederick's darts.
Almost a Throw It Out moment; in some interviews, Brooks stated that the only point during production where he and co-writer Gene Wilder seriously disagreed was the inclusion of the "Puttin' On the Ritz" number. Gene loved the idea but Mel hated it. After Gene vehemently defended the scene, Mel decided, "If you feel that strongly about it, we'll shoot the scene. If it works, we'll use it, if not, we won't." They shot it and it became one of the highlights of the movie.
The monster's shout of "PUTTINAHNDARIZZZ!" was Peter Boyle's idea.
Wilder wrote in his autobiography that he kept trying to find ways to add additional scenes to the shooting schedule because the cast had a thoroughly great time together and Wilder didn't want filming to end.
"Soitenly! You take the blond, I'll take the one in the toiban!" Marty Feldman's Groucho Marx-esque riff was one of many moments that Mel Brooks was not expecting, to his delight.
The blind man's parting line "I was going to make espresso!" was improvised by Gene Hackman. The scene immediately fades to black to cover up the laughter of the film crew.
What Could Have Been: Originally in the script, after Igor drops the brain jar, he was to turn to the camera and quip, "Funny thing is, I tried!"