Season 2, Episode 6 - Halloween
This slip-up occurs during the Halloween episode, where Ethan Raine makes his first appearance as a seasonal costume shop owner who curses everyone in town who shopped at his store to turn into their costumes. Willow and Giles visit Ethan's shop to confront him and try to stop the spell. At this point Willow is a ghost-version of herself, because she had dressed up as a "spooky" ghost to take some kids trick-or-treating.
When Giles sees Ethan and realizes things are probably going to get ugly (translation: Giles is gonna kick some fellow-Brit arse!), he firmly tells Willow to leave the shop immediately. She runs off camera and moments later, you can hear the shop door open and shut as Willow exits.
This is clearly an error, as it was made clear in earlier scenes that Willow, in her ghostly state, was completely unable to touch or move any tangible objects or beings... Xander walked right through her, and she was even seen entering and exiting other rooms by simply walking into the wall/door. Ergo, she wouldn't have had the need for nor ability to use a door.