SpongeBob Meets the Strangler / Pranks a Lot - S3-E20
Continuity mistake: In "Pranks-A-Lot" when Sandy flies out of her dome in an escape pod to Texas, she leaves a big hole on top of her dome. In the next shot, it's gone. Very noticeable mistake.
Mermaid Man and Barnacle Boy IV / Doing Time - S3-E1
Continuity mistake: In the scene where the guard pulls Mrs. Puff out of the rock yard, her hat is red, then blue, then red again throughout the cuts in the scene.
Chocolate With Nuts / Mermaid Man And Barnacle Boy V - S3-E11
Revealing mistake: In one scene with Spongebob, Patrick, an old lady, and her almost dead mom, in one part Spongebob has no holes on his face. There is also a light-colored spot near Patrick's mouth.
Mermaid Man and Barnacle Boy IV / Doing Time - S3-E1
Other mistake: When the citizens pop out of Sponge Bob's holes near the end of Mermaid Man and Barnacle Boy 4, if you look closely, Sandy's head is gone except her eyes and nose.
Answer: I believe this is the Season 6 episode "Spongicus".