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.
The Snowball Effect / One Krabs Trash - S3-E3
Other mistake: One Krab's Trash: When Mr. Krabs sticks the ghost drawing in Spongebob's window with the fishing rod, the shadow of the paper is visible on Spongebob, but there is no shadow of the fishing line holding it up. (00:17:27)
The Snowball Effect / One Krabs Trash - S3-E3
Other mistake: In 'The Snowball Effect' when Squidward attempts to make Patrick and Spongebob resume their snowball fight he throws a snowball at them which they avoid, and it puts out Squidward's fire inside. Squidward had his back towards his house and threw the snowball in the opposite direction.
SpongeBob's House Party (aka Party Pooper Pants) - S3-E10
Other mistake: When Patchy pops Minnie Mermaid's pool, water comes out from a different place.
Krabby Land / The Camping Episode - S3-E19
Other mistake: In "Krabby Land", Mr. Krabs as "Krabby The Clown" finishes his short speech and goes into his office. When he slams the door, bubbles float up and those bubbles stay motionless in the air for a few frames.
SpongeBob Meets the Strangler / Pranks a Lot - S3-E20
Other mistake: Pranks a Lot: When everyone is cheering on Spongebob and Patrick at the end, Sandy whistles by putting her hand through her helmet. Impossible. (00:23:30)
Answer: I believe this is the Season 6 episode "Spongicus".