Best Day Ever/The Gift of Gum - S4-E20
Revealing mistake: When Squidward says "I suppose if you haven't fixed my clarinet, the concert would have been a bust", Sandy's tail is the same color as her suit.
Best Day Ever/The Gift of Gum - S4-E20
Character mistake: SpongeBob tries to steal the gum off his door at night. The next day, SpongeBob says that it was still Best Friends Day, so he would give Patrick his gum back. Really, it should have been the next day.
Best Day Ever/The Gift of Gum - S4-E20
Continuity mistake: When SpongeBob tries to steal the gum at night, he's dressed as a bandit, but then the next day when Sandy finds him, he's wearing his normal clothing. He was stuck in the gum all night so there's no way he would have changed.
Answer: I think that this was supposed to be an intended joke. Patrick is too stupid to remember or achieve most things and SpongeBob is able to remember and achieve most things, but the one thing that SpongeBob cannot achieve, is the one thing Patrick can achieve on the first go.
Casual Person