That happens when they use a template background as a standard for an event that hasn't happened yet. Mistakes like this happen a lot when you create those memory kind of episodes where you go back in time to remember an even that is already current in the show.
While the quality of the Simpsons has been argued and debated to have gone down after so many seasons, one thing that has always stayed pretty consistently fun and entertaining is the Couch Gags and Treehouse of Horror episodes. At least for many anyway. I haven't admittedly kept up with the show in quite some time, so perhaps this has changed.
Edited: Thomaslee on 24th Jul, 2022 - 4:49pm