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.
The Dark Network of Etemon - S1-E15
Revealing mistake: Watch the scene where Biyomon and Palmon are eating. A Pagumon passes, Biyomon turns her head and then has two beaks. Obviously a frame overlap.