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.
Gullible Pants/Overbooked - S6-E19
Other mistake: In the episode where SpongeBob has to do 3 things in 1 night, near the end, they all get frustrated for not doing their tasks. Look at Mr. Krabs. He runs over to SpongeBob and behind him is bubbles. Impossible, because in Sandy's dome is air.
Continuity mistake: When Mr. Krabs' ancestor eats SpongeBuck's chilli he takes a spoonful and spits it out. In the next shot of SpongeBuck, chilli is all over is face, but he didn't spit out that much chilli to have his entire face covered.
Pressure / The Smoking Peanut - S2-E12
Audio problem: In the beginning of "The Smoking Peanut", Spongebob takes his hat and spits it in the air like Clamu would do. Before he puts the hat in his mouth, he says, "a cannonball", the camera is on Spongebob and his mouth isn't saying that. He has his mouth open to put the hat in his mouth.
Pressure / The Smoking Peanut - S2-E12
Continuity mistake: After SpongeBob confesses he threw the peanut at the oyster, a zoo keeper says, "wait, here is the real criminal", and he drags out Mr.Krabs. If you look at his pockets you can see food, candy, and other junk. Yet when the zoo keeper accidentally rips Mr.Krabs clothes off the pockets are completely empty.
The Chaperone / Employee of the Month - S1-E12
Continuity mistake: When Spongebob is talking about Annoy Squidward Day, you see a calendar with 28 days, indicating that it's February, but the empty picture on the wall says January.
Mermaid Man and Barnacle Boy III / Squirrel Jokes - S2-E4
Continuity mistake: In "Squirrel Jokes", on Spongebob's second act when Sandy laughs about the bronze fish, she is between 2 people. When you see her again she isn't between anybody.
Audio problem: In the scene where Spongebob is falling into Gary's dream, Spongebob is screaming for at least one second after he hits the ground.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: In Jellyfishing, when SpongeBob and Patrick first take their nets to jellyfishing, they only take their own nets. But when they go to Squidward's house, they have another net, just for him.
The Graveyard Shift / Krusty Love - S3-E13
Continuity mistake: In "The Graveyard shift", Squidward begins to count the signals that means the ghost will appear. "First the lights, then the phone.", and a green slime appears in the wall. Squidward says "Oh, wait, they always do that, but what was the third thing?", and the bus appears at the front of the Krusty Krab. SpongeBob says, "I didn't know the bus comes here at this hour", and look at the wall: the green slime has disappeared.
Bubblestand / Ripped Pants - S1-E1
Continuity mistake: In the scene where Squidward complains to Spongebob about the construction noise from his stand, Squidward's porthole in the background is black, but from the interior there is a light on (Note: this also happens in every other shot of his house.
Bubblestand / Ripped Pants - S1-E1
Continuity mistake: SpongeBob takes off his ripped pants, and throws them on the ground. His shirt sleeves are perfectly intact. Next, Sandy and Larry are shown playing volleyball. When SpongeBob is shown again, his pants are still gone, and so are his shirt sleeves.
Survival of the Idiots / Dumped - S2-E8
Continuity mistake: In the episode "Survival of the Idiots," when Sandy wakes up in Spring and goes outside, the first time we see Patrick and SpongBob standing side by side, SpongeBob is to our left, but later when SpongBob is trying to make up an excuse, they've swapped places.
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.
Answer: I believe this is the Season 6 episode "Spongicus".