Watch when Plankton/Sponge bob is walking to Squidward's house.Sponge bob breaks through Squidward's wall but hes upstairs? Squidwards bedroom is upstairs!!! Sponge bob walks out a bit later and hes down staires.. Oops!
This is not a slipup. One requirement states that you must have no idea of any other logical explanation for it. He could have had a guest room downstairs that he sleeps in. After all, you never see downstairs.
HAH! I was just about to post this slipup. Great job!
And daunrealist, you see the downstairs in MANY episodes ((for example, "Nautical Neighbors" where the downstairs of his house has many rooms, including a living room/kitchen)). And even if he had a guest room downstairs, I don't think it would take up the entire downstairs of his house ((which it did in this case)).
you also see the lower level in house fancy, the one where spongebob and patrick have dinner (well... just the soda pop) and the one where squidward tries to enjoy his sunday.