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.