In the episode 'Simpson Tide' in the scene where Homer is lying on the couch after just being fired by Mr Burns for setting the power plant alight by trying to make a donut bigger. He's talking about being fired.

Lisa is sat infront of the couch reading a Junior Skeptics Magazine. She is clearly in her pyjamas, which is a blue nighty and slippers.

Then Homer watches a commercial for 'The Naval Reserve'. Half way through this when the commercial says 'Mississippi National Guard and The League of Woman Voters' Lisa is still in her pyjamas.

The commercial then continues and at the end it pans back to Homer and Lisa and Homer says how he's 'taken alot from this country' and Lisa is in her normal every day clothes!!! It's been barely 5 seconds and she's changed! Is there some sort of clothes changing machine we need to know about?