I was watching the Season 1 deleted scenes and noticed this. being that the entire original take of "Some Enchanted Evening" was REALLY off model, its meant that they made a few mistakes in animation. So this is one of four times in the simpsons when you actulley see that she does wear underwear If you want to see the other 3 just ask and I will post them
I found these photos while packing today. All but the bottom right, which you've already seen: [link]
These are all photos taken by me at the local fair around 1990-91, when I was 9 or 10 years old. It was a free show, and the fairgrounds were within walking distance of my house, and being a huge Simpsons fan back then (WAY bigger than I am now, believe it or not) I begged my mom to take me to the show, and she did.
I have no idea what book Homer is reading in the top left photo. The top right photo with Homer, Marge and Lisa looks like it must have been a musical/dance number. The bottom left shows Marge and Homer talking to Principal Skinner, with Bart hiding in the background. FYI Skinner wears a wig! Bart steals it as a prank, if I remember correctly. And the bottom right is a close-up of my then-idol, Bart, back when he wore "The Mysterious and Legendary BLUE Shirt."