Mermaid Man and Barnacle Boy / Pickles - S1-E6
Continuity mistake: Mermaid Man and Barnacle Boy: When Barnacle Boy opens the box with the two rings inside, in the closeup the "B" ring is on the left and the "M" ring on the right, but in the wideshot Barnacle Boy removes the ring on the left (the "B") and places it on Mermaid Man's finger, then in the next shot we see it's the "M" ring, before both rings vanish.
Mermaid Man and Barnacle Boy / Pickles - S1-E6
Continuity mistake: Mermaid Man and Barnacle Boy: When SpongeBob and Patrick (dressed in their superhero costumes) exit the pineapple the front door opens at the right side (outward), but after getting on Squidward's nerves, when they retreat back inside the front door opens the opposite way at the left side (inward).
Dying For Pie / Imitation Krabs - S2-E6
Continuity mistake: Mr Krabs has no eyelashes when he tells Squidward not to tell Spongebob about the bomb he ate. (00:05:01)
FarmerBob/Gary and Spot - S12-E1
Continuity mistake: In "FarmerBob" when SpongeBob is taken to Old Man Jenkins' farm, after that, the next shot when he is stretched about 50m tall by a bag of potatoes, the tie is missing. (00:00:50 - 00:01:25)
Grandma's Kisses / Squidville - S2-E9
Continuity mistake: When Spongebob and Patrick first arrive at the crowd, there's a window on the wall next to them. After they mistake that one octopus for Squidward, the window has vanished.
Driven to Tears/Rule of Dumb - S4-E17
Continuity mistake: In 'Rule of Dumb' Patrick's house returned to its former self in the outside shot just before Squidward's house was moved.
The Thing/Hocus Pocus - S4-E16
Continuity mistake: In 'The Thing' the house of Squidward's door first opened outwards, then later inwards. It also changes color.
Chocolate With Nuts / Mermaid Man And Barnacle Boy V - S3-E11
Continuity mistake: In 'Chocolate With Nuts' the bandaged up fish's house's front stairs had only a few steps when Spongebob and Patrick approached, but as the fish falls down there's suddenly a lot more.
Krab Borg / Rock-a-Bye Bivalve - S3-E8
Continuity mistake: In 'Rock-a-Bye Bivalve' Spongebob's front door changes hinge sides. Also at the end suddenly there's a light switch next to the door which wasn't there earlier.
The Secret Box / Band Geeks - S2-E14
Continuity mistake: In Band Geeks on day four the lever besides the big gate changes positions between pointing upwards and downwards several times.
Scaredy Pants / I Was a Teen-age Gary - S1-E13
Continuity mistake: Spongebob's belt is missing when his clothes are first revealed while dangling in midair.
Scaredy Pants / I Was a Teen-age Gary - S1-E13
Continuity mistake: The Krusty Krab is filled with Halloween decorations during the episode, but when Spongebob jumps to hide in a barrel there are no decorations in sight.
Scaredy Pants / I Was a Teen-age Gary - S1-E13
Continuity mistake: When Spongebob agrees to hear Mr. Krabs' story, their seats come out of nowhere.
Selling Out/Funny Pants - S4-E5
Continuity mistake: In "Selling Out", Mr. Krabs hears the footsteps of many customers approaching and the shot shows the entrance to the Krusty Krab from the inside, with no one in sight. When Mr. Krabs tells the guys that they're coming, it shows Squidward in a few shots mopping right in front of the doors, even though he wasn't seen there previously.
Continuity mistake: Plankton is questioning Mr. Krabs and SpongeBob for the formula whilst they are stuck inside a mechanical hand. The area where the screw is located can be seen on both sections of the hand, on the right and left hand side. In some shots the right side of the hand completely obscures the screw on the left side, and in some shots not. Also, in one shot, the screw changes colour and becomes white.
Patrick's Coupon/Out of the Picture - S10-E10
Continuity mistake: In "Patrick's Coupon", after Patrick is released from the crane, he suspects two male fish of wanting to steal his coupons. Next, he hears two female fish talking and thinks they want his coupon too. When he goes to talk to them, the guys he was just talking to have disappeared. In two shots, the male fish reappear.
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.
Continuity mistake: In the episode "Plankton's Army", there is a group of Plankton's family sitting on top of the toilet when Mr. Krabs is first shown inside it. In two shots, the group has vanished from the top of the toilet.
Continuity mistake: After Plankton blows up the Krusty Krab roof, Mr. Krabs enters the kitchen through the door leading from his office. When he opens the door, it falls off and leans against the counter beside the door. When SpongeBob approaches Mr. Krabs, the door is suddenly lying on the floor.
Answer: I believe this is the Season 6 episode "Spongicus".