Ghost Light - S3-E6
Continuity mistake: When the three are arguing in the dressing room, Oliver says, "I don't care," then turns and wipes most/all of the lipstick writing off the mirror, leaving a load of messy smears behind. In the shot of Oliver when he says, "in a few weeks..." the mirror is behind him, with no smears. At the end of the scene as Mabel says "I'm done" and storms out, the mirror is very smeary again.