The Secret Box / Band Geeks - S2-E14
Audio problem: At the end of "Band Geeks" when everyone is playing "Sweet Victory" at one point Mrs. Puff is shown playing the guitar. When this happens, you can see the others in the background not doing anything, even though their instruments are heard.
The Secret Box / Band Geeks - S2-E14
Continuity mistake: When SpongeBob is sneaking to Patrick's house, Squidwards house is missing.
The Secret Box / Band Geeks - S2-E14
Continuity mistake: In Band Geeks on day four the lever besides the big gate changes positions between pointing upwards and downwards several times.
The Secret Box / Band Geeks - S2-E14
Revealing mistake: When Squidward asks who has played instruments before and Plankton responds, look at Plankton's triangle. The bottom disappears for a split-second.
The Secret Box / Band Geeks - S2-E14
Continuity mistake: In Band Geeks during the Day Two scene, in the beginning you can see Pearl near the front, but in the distant shot that follows Pearl has moved further back. (00:04:20)
Answer: I believe this is the Season 6 episode "Spongicus".