After the Roquefort Diamonds get dropped into the vault, some enemies suddenly spawn in places where they were and weren't there previously. But once you kill them, they will never respawn again. Also, if you roll down the bowling ball in the rocket exhibit upstairs, it will respawn a Brown Bear on the ramp.
It is unknown how often this glitch occurs and how it is caused, but in Viggo's Revenge, shooting the security guard may cause the music to stop playing. It will start back up when you switch fighters.