A Flea in Her Dome / The Donut of Shame / The Krusty Plate - S5-E11
Continuity mistake: In "A Flea in Her Dome", Patrick asks Spongebob to get on Sandy's shoulders. Then, he climbs onto Spongebob's shoulders. In the next shot, they are suddenly next to Sandy's tree.
A Flea in Her Dome / The Donut of Shame / The Krusty Plate - S5-E11
Continuity mistake: In "A Flea in Her Dome", Patrick gets bitten by a flea and his chest is covered in bites. He takes out a pencil and connects some of the dots to make a horse. A few shots later, the horse and the bites disappear.
Money Talks / SpongeBob vs. The Patty Gadget / Slimy Dancing - S5-E10
Audio problem: In "Slimy Dancing" in the scene where Squidward and Spongebob perform their act, we first see Squidward making Spongebob spin around. There's a shot where it shows Squidward inside Spongebob. The curtain in the background in the shot remains still, even though you can hear spinning.
Whatever Happened to SpongeBob? (WhoBob WhatPants) - S5-E18
Continuity mistake: Near the beginning, Spongebob picks up Gary, and some of his slime falls on the floor. When he hugs him and breaks his shell, the slime is gone. (00:01:56)
Picture Day/Pat No Pay/Blackjack - S5-E13
Continuity mistake: When Patrick picks up the fridge and throws it on the barrel, there is some metal on the floor. When Mr. Krabs walks into the kitchen 2 seconds later, there is no metal on the floor.
A Flea in Her Dome / The Donut of Shame / The Krusty Plate - S5-E11
Factual error: In 'A Flea in Her Dome' Sandy opened up the door for water to fill her dome. The water even replaced the air on the inside which was above the door, which isn't possible because the air would have to go down through the door to escape the dome.
Spy Buddies / Boat Smarts / Good Ol' Whatshisname - S5-E4
Continuity mistake: In "Good Ol' Whatshisname", Squidward is seen marking his calendar which has five weeks on it, and each row has a set of numbers. In the next shot, when it shows the rest of Squidward's jail cell, the bottom two rows of the calendar are completely blank.
Spy Buddies / Boat Smarts / Good Ol' Whatshisname - S5-E4
Continuity mistake: In "Good Ol' Whatshisname", Squidward uses a pencil to mark his calendar and sets it down on his bench when Mr. Krabs and Spongebob come to visit them. He goes up to the bars, and in a few shots, when it cuts to him, you can see the bench he was just on and the pencil has disappeared.
New Digs / Krabs à la Mode - S5-E6
Continuity mistake: In "Krabs à la Mode", Spongebob is wearing miniature mittens on his eyelashes while walking to the Krusty Krab. They disappear after a few shots.
A Flea in Her Dome / The Donut of Shame / The Krusty Plate - S5-E11
Continuity mistake: In "The Krusty Plate", Spongebob finds that one plate has a brown smudge on it. After an attempt to clean the plate, he lifts it up and the smudge is now a light shade of green.
The Original Fry Cook / Night Light - S5-E9
Continuity mistake: Near the end of "Night Light" Patrick gets burned by the sun, and the wooden stem atop his house is missing.
Rise and Shine / Waiting / Sing a Song of Patrick - S5-E1
Continuity mistake: In "Waiting", SpongeBob has milk on his face. It then cuts, and the milk is not on his face anymore.
Rise and Shine / Waiting / Sing a Song of Patrick - S5-E1
Factual error: As Patrick's song causes havoc, two airplane pilots deserts the airplane and deploy their parachutes. The issue with this is that the two pilots should have come to a stop relative to the airplane's movement.
Roller Cowards / Bucket Sweet Bucket - S5-E8
Deliberate mistake: In 'Bucket Sweet Bucket' there's no way that Plankton managed to reach with his paint brush to repaint the entire front of his establishment from the top of Squidward's head.
Banned in Bikini Bottom/Stanley S. SquarePants - S5-E20
Continuity mistake: In Stanley S. Squarepants, when SpongeBob and Stanley go see Squidward when he's getting the newspaper, both Squidward and the newspaper change position in between shots.
Answer: I believe this is the Season 6 episode "Spongicus".