Hooky / Mermaid Man and Barnacle Boy II - S1-E20
Continuity mistake: When Spongebob takes his outer suit/pants off and finds out that the hook is stuck through his underpants, Mr. Krabs places a Help Wanted sign up on the front door of the Krusty Krab. When Spongebob crashes onto the doors naked, the sign disappears.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: In 'Plankton', when Spongebob is being controlled by Plankton and he bounces into the wall in the Chum Bucket, he is tumbling in a way so that when he is standing, he is facing away from the ingredient identifier. When the shot changes, Spongebob is now facing the identifier while standing.
Your Shoe's Untied / Squid's Day Off - S2-E2
Continuity mistake: In "Squid's Day Off" when Mr. Krabs is washing the money that came out of the register, Spongebob taps him on the shoulder while the faucet is still on. When Mr. Krabs drops the dime into the sink, the water turns off without anyone touching the handles.
SpongeBob Meets the Strangler / Pranks a Lot - S3-E20
Continuity mistake: In "Pranks A Lot": SpongeBob and Patrick are going to burn Mr. Krab's dollar, so he throws water on them and they become visible. Patrick is holding Mr. Krab's dollar. When he says "We're visible again", and he and SpongeBob start to scream, the dollar completely disappears.
Something Smells / Bossy Boots - S2-E1
Continuity mistake: In Something Smells, when Spongebob is playing the organs at night, the level of ambient light increases dramatically between the first and second shots.
SpongeBob's House Party (aka Party Pooper Pants) - S3-E10
Continuity mistake: When Spongebob comes home after being arrested, he picks a spare key up from under his mat and gets angry at himself for trying to break in the night before. When he walks in, discomfort on his face, the mat is gone.
Grandma's Kisses / Squidville - S2-E9
Audio problem: After SpongeBob causes a flood in his grandma's house, he cries because he is fed up of being an adult and his grandma tells him he doesn't have to be a baby to get her love. When SpongeBob replies, "I'm not?" note that in his reflection, his mouth does not move.
Dying For Pie / Imitation Krabs - S2-E6
Continuity mistake: Before Spongebob drops the pie in Squidward's face, the rock he tripped on was never there.
Something Smells / Bossy Boots - S2-E1
Continuity mistake: In Something Smells, when Spongebob is making the sundae, the peanut plant vase that he throws in changes direction between shots.
The Great Snail Race / Mid-Life Crustacean - S3-E15
Continuity mistake: When Gary was spinning out of control, there were at least 5 lanes on the track but before there were only 3.
You Wish ('Shanghaied') / Gary Takes A Bath - S2-E11
Continuity mistake: When Gary almost goes into the mud and his head is hovering over it, in the far shot, he is just sitting there.
Survival of the Idiots / Dumped - S2-E8
Continuity mistake: In Dumped, when Patrick is finished brushing his teeth, his toothbrush disappears.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: The words "Total Control" constantly appear and disappear from the controlling device in Spongebob's brain.
Something Smells / Bossy Boots - S2-E1
Continuity mistake: When Spongebob jumps on a car and asks "am I ugly", and the guy blows up his car, you see his car has small little wheels. When one of the wheels bounces on Spongebob, it is huge.
Fools in April / Neptune's Spatula - S1-E19
Continuity mistake: When Spongebob grabs Squidward's legs you only see him grabbing at two of Squidward's legs and the other two legs have disappeared, but when the shot changes they're back again.
Your Shoe's Untied / Squid's Day Off - S2-E2
Audio problem: In "Your Shoe's Untied" when Gary ties Spongebob's shoes he says "hoppin' clams where'd you learn to do that?" When he says "clams" his lips are out of sync.
The Chaperone / Employee of the Month - S1-E12
Audio problem: We see Spongebob in his long legs walk over to Mr. Krabs' house, then we see the door from inside the house. Through the window we see Spongebob's long legs, yet we hear him spring boing his way over when we see them already there.
No Weenies Allowed / Squilliam Returns - S3-E6
Continuity mistake: When Squidward has sort of a strict conversation with Spongebob, Mr. Krabs, and later Patrick, the 3 of them he was talking to, were side by side, but a shot later, Mr. Krabs and Spongebob are side by side, but farther from Patrick.
The Sponge Who Could Fly (aka The Lost Episode) - S3-E16
Continuity mistake: After the walk cycles, the color bars change between shots.
Nasty Patty / The Idiot Box - S3-E4
Continuity mistake: In the scene where Spongebob and Mr. Krabs bury the health inspector, Spongebob buries him so that way his head is above ground covered by a pile of dirt. In the next shot of the buried health inspector, he is buried horizontally.
Answer: I believe this is the Season 6 episode "Spongicus".