Check out Buffy's jumping back pack in the episode "Phases".

Buffy enters the Bronze trying to catch the werewolf, just as everyone else has come running out. You can clearly see that her journey from the door to the backstage entrance has been made up from 3 or 4 different takes, since the strap on her backpack keeps sliding up and down her shoulder, one minute being firmly on both shoulders, then hanging down on one side, then back up and finally, only over one shoulder.