Jellyfishing / Plankton! - S1-E3
Other mistake: In "Jellyfishing", when SpongeBob and Patrick walk back to their houses covered in bandages, when SpongeBob says "Hey, Patrick." Patrick is mouthing what SpongeBob says.
Mermaid Man and Barnacle Boy / Pickles - S1-E6
Other mistake: In the end credits for the episode "Pickles", Bubble Bass is named "BubbleBass" (without a space).
Sandy's Rocket / Squeaky Boots - S1-E8
Other mistake: In the episode "Sandy's Rocket", Spongebob and Patrick go "alien-hunting." SpongeBob and Patrick are in Squidward's house. But when SpongeBob holds the hot water bottle, and he says "No, it's an egg-sack!", one of the drops leaking from the bottle goes up instead of down.
Nature Pants / Opposite Day - S1-E9
Other mistake: When SpongeBob throws the fire extinguisher at the grill, the patties are shifted to the left and appear to be floating.
MuscleBob BuffPants / Squidward, the Unfriendly Ghost - S1-E11
Other mistake: When Squidward is on the royal bed, Spongebob's Pineapple is nowhere in sight.
The Chaperone / Employee of the Month - S1-E12
Other mistake: When Pearl is crying in the Krusty Krab, the customers are just the same person over and over and over.
SB-129 / Karate Choppers - S1-E14
Other mistake: SB-129: When Squidward goes inside the freezer and shuts the door, the freezer's door handle vanishes mid-shot. (00:03:09)
Other mistake: Rock Bottom: When SpongeBob sets up a cardboard version of himself and is shown sneaking to the bus, his holes are missing and he is perfectly square, instead of his usual sponge-like shape. (00:18:47)
Answer: I believe this is the Season 6 episode "Spongicus".