The Great Snail Race / Mid-Life Crustacean - S3-E15
Continuity mistake: When SpongeBob runs to help Gary in the race, Patrick's "snail" has disappeared.
SB-129 / Karate Choppers - S1-E14
Continuity mistake: When Squidward is in the white place and crashes through the floor, he doesn't have his clarinet before he crashes through the floor. But when he is in the time machine, he has his clarinet.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: In "Plankton", when Plankton says tomorrow is his birthday, he is near Spongebob's thumb. When the camera zooms in on Plankton, he is instantly in the middle of Spongebob's hand.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: At the beginning of Plankton., Plankton is very small. Later in the episode, as in the rest of the show, he's bigger and not that small.
Your Shoe's Untied / Squid's Day Off - S2-E2
Continuity mistake: When SpongeBob is sad because he can't tie his shoes, you see is night. When Gary ties his shoes, suddenly it becomes day.
The Chaperone / Employee of the Month - S1-E12
Continuity mistake: When Spongebob, in his black suit and long legs, is at the door of Mr Krabs', he used a lever to lower himself down. When we cut back to Spongebob, it's gone.
Mermaid Man and Barnacle Boy III / Squirrel Jokes - S2-E4
Continuity mistake: In "Squirrel Jokes", SpongeBob wants to make a joke about a fork. Someone screams "Forks?" and SpongeBob becomes more nervous. After that, the fork disappears from where it was.
Your Shoe's Untied / Squid's Day Off - S2-E2
Continuity mistake: In "Squid's Day Off", sometimes Squidward's house door opens to the left, and sometimes opens to the right.
The Bully / Just One Bite - S2-E17
Continuity mistake: In "Just One Bite," when everyone says "The only people who don't like Krabby Patties have never tasted one," the kitchen doesn't have a back door. After Squidward says "I gotta have more," he looks in from the back door.
The Sponge Who Could Fly (aka The Lost Episode) - S3-E16
Continuity mistake: When the parrot lands the egg on the remote, in the next shot it vanishes.
Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Continuity mistake: At the beginning of "Born Again Krabs", it shows the outside sky being orange-ish. But when it shows SpongeBob and Mr. Krabs talking about the old patty, you can see a little blue crack through the window.
Rise and Shine / Waiting / Sing a Song of Patrick - S5-E1
Continuity mistake: In "Sing a Song of Patrick", Patrick creates his own song and listens to it with Spongebob in his pineapple. And at the end of the song the pineapple melts and the stereo falls on top of them and not the pineapple. But when they get up, the busted stereo disappears and the pineapple is on top of them.
Grandma's Kisses / Squidville - S2-E9
Continuity mistake: In Squidville, Spongebob and Patrick are at the front gate and when Spongebob says, "But you just ate 3 orders of fried oyster skins," Spongebob's teeth disappear.
The Chaperone / Employee of the Month - S1-E12
Continuity mistake: When Spongebob is crying next to the employee of the month wall, (just before he says "How could I have let the quality of my work slip so much?") the order of the pictures is messed up. It goes December, February, January. Then, when we see the close-up of Squidward's face laughing, the order is correct again.
Fools in April / Neptune's Spatula - S1-E19
Continuity mistake: Eventually after Spongebob takes the door wheel handle off, it comes back.
Spy Buddies / Boat Smarts / Good Ol' Whatshisname - S5-E4
Continuity mistake: At the beginning of Spy Buddies, when Mr. Krabs is entering the code to enter the Krusty Krab, it's night. But when Patrick yells about him wanting to be a spy and it shows the fish walking outside of the Krusty Krab, it is day.
The Algae's Always Greener / Sponge Guard On Duty - S3-E7
Continuity mistake: In "The Algae's Always Greener", after Squidward barges into Mr. Plankton's office, you can see that there are no krabby patties on Mr. Plankton's desk, but when it pans back to Mr. Plankton's desk, there are two krabby patties on his desk. (00:07:35)
Continuity mistake: When we see the "Flabby Patty" for the first time, there are 8 fish in line. When we see it the second time, there is only 5.
Continuity mistake: Sometimes there is a window on the right side of Squidward's house, sometimes there's not.
Survival of the Idiots / Dumped - S2-E8
Continuity mistake: In "Dumped", when Gary takes Patrick's cookie they are in the laundry room. When SpongeBob wants to take Gary for a walk, they instantly appear in the living room. SpongeBob also has a leash in his hand. There is no time for them to walk from the laundry room to the living room.
Answer: I believe this is the Season 6 episode "Spongicus".