There's actually 2 slip-ups in this episode.

1. While Lisa is at the power plant with Homer, they try to make things more fun by pretending radiation suits are space suits. They then go spinning in circles on chairs while in the suits. If you watch their faces, you'll see that their mouths aren't moving yet you hear them laughing.

2. For the second you have to watch two parts of the episode. When Lisa first asks what the radiation suits are they're on the left wall (left if you're facing Homer's console). But later on when Homer sees them and is reminded of when he and Lisa were playing around with them they're on the right wall. I suppose Homer and Lisa could've hung them up on the other wall after playing, but what would be the of having hooks to hang them on on both walls?