This slip-up was in the episode where Spongebob thinks he lost his name tag. When Spongebob is going through what happened that day (after Patrick said "hi"), there are one or two events in his day that you see from behind. Since Spongebob put his pants on backwards, you should see the pants backwards, too. But, if you look closely to the back of Spongebob during those events, you don't see his little collar flaps on the back.

And before anyone says it, his pants were backwards the entire episode, you don't see the collar flaps in front either.