At the end of Gift of Gum, Spongebob tells Patrick that "It's still Best Friend's Day" when, in fact, it is the next day (the "theft" attempt takes place at night).
At the end of the episode, Sandi's tail is white in one shot while she's on stage, but in the next shot it's back to brown.
When Sponebob was on the stage, Sandy's tail is white. When it's turned to Spongebob and back, Sandy's tail is now to normal.
At the concert when they roll out the grill full of krabby patties, there are 6 patties on the grill.
When they roll the grill off the stage there are 7 krabby patties on the grill.
SpongeBob pulls one of his front teeth to replace Squidwards broken reed on his clarinet, but in the very next scene he has 2 front teeth again.
At the end of the episode, when Spongebob is singing "Best Day Ever", you can see Krusty Krab written behind him. In the next shot it says K.K.