Revealing mistake: Right before Doctor Frankenstein (Gene Wilder) gives life to his creation he is standing behind the monster on the platform giving his speech. While he is giving his speech you can see the "dead" monster breathe, not only once but several times.
Revealing mistake: For most of the opera house stage presentation, the monster is a full head taller than Friedrich. During the tap dance, however, they're the same height. Although the camera angle/depth of field used tries to disguise this by keeping Gene Wilder in the foreground for most of the shots, it's still apparent that Peter Boyle isn't wearing his probably-impossible-to-dance-in platform shoes during the sequence.