As Seen on TV / Can You Spare a Dime? - S3-E5
Continuity mistake: As Seen on TV: After SpongeBob shouts about the grand finale he tosses all the food at the hungry crowd, and as they all happily exclaim, "Krabby Patties," there is only one door to the right of Squidward's order station in this shot, though there should be two doors.
As Seen on TV / Can You Spare a Dime? - S3-E5
Continuity mistake: As Seen on TV: The front door to Mr. Krabs' office is a solid door with an anchor on the exterior side of the door, but in a few shots the office's front door has a porthole window like other doors in the restaurant.
Krab Borg / Rock-a-Bye Bivalve - S3-E8
Continuity mistake: Krab Borg: The door to Krabs' office has a porthole in many shots, but after Krabs burns his eyes he rushes out of his office into the eating area, and when he runs past Squidward and SpongeBob the office door is the normal solid door, with an anchor on the exterior side.
Chocolate With Nuts / Mermaid Man And Barnacle Boy V - S3-E11
Continuity mistake: Chocolate With Nuts: While SpongeBob and Patrick are selling chocolate, the elderly female's door opens at the left, but after Patrick claims their chocolate will make you live forever, the door is open the opposite way.
Chocolate With Nuts / Mermaid Man And Barnacle Boy V - S3-E11
Continuity mistake: Mermaid Man And Barnacle Boy V: When Barnacle Boy shouts that he wants to be called Barnacle Man and demands respect, the door to Krabs' office is missing. Then it happens again when the news comes on.
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.
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.
Krabby Land / The Camping Episode - S3-E19
Other mistake: In "Krabby Land", Mr. Krabs as "Krabby The Clown" finishes his short speech and goes into his office. When he slams the door, bubbles float up and those bubbles stay motionless in the air for a few frames.
Krabby Land / The Camping Episode - S3-E19
Continuity mistake: In "The Camping Episode", Squidward sets down a blanket to lay under the stars and in a couple of shots, the blanket disappears.
Krabby Land / The Camping Episode - S3-E19
Continuity mistake: In "Krabby Land", Spongebob goes to check his motivational quote and from far away, the page is flipped up. But in the next shot, it is a close up and the page is flat against the stack. In the following shot, the page is flipped up again.
Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Continuity mistake: In "I Had An Accident", Sandy opens the door to show that she was being "attacked" by a gorilla and Spongebob is looking through that door to see the events unfold. But after a couple of shots, the door has closed by itself and Spongebob has to open it again before going outside.
Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Continuity mistake: In "Born Again Krabs", when Mr. Krabs first makes his appearance next to Davy Jones' locker, there is some coral taking up a part of the bottom left corner. In a few shots, that same set of coral has disappeared.
Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Continuity mistake: In "Born Again Krabs", after Mr. Krabs returns from the hospital and the Krusty Krab has their "Grand Re-Opening", there are two big yellow signs on the front windows, but in the next shot from the inside, these signs have vanished.
The Graveyard Shift / Krusty Love - S3-E13
Continuity mistake: In 'The Graveyard Shift' Squidward explains that there are three signs concerning the arrival of the Hash-Slinging Slasher. He then holds up three hands. In the next shot, he is only holding up one hand.
Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Continuity mistake: In "Born Again Krabs", the doctor taking care of Mr. Krabs drops his clipboard and runs. When the Flying Dutchman appears in the next shot, the clipboard is missing.
Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Continuity mistake: In "Born Again Krabs", Mr. Krabs drops the coins that the Flying Dutchman gave to him on the floor. In a few shots, when the Flying Dutchman comes back, the coins have vanished.
Chocolate With Nuts / Mermaid Man And Barnacle Boy V - S3-E11
Continuity mistake: In "Chocolate With Nuts", Patrick and Spongebob buy the chocolate holding bags from one man, they walk to the next house next door. When they are standing next to the door, the first house behind them disappears.
Club SpongeBob / My Pretty Seahorse - S3-E12
Continuity mistake: In "My Pretty Seahorse", Spongebob is walking outside with Mystery and a customer with a drink and a plate is watching him go by. In the next shot, Squidward places a bowl of onions underneath his face, and his plate and drink have disappeared.
Club SpongeBob / My Pretty Seahorse - S3-E12
Continuity mistake: In "My Pretty Seahorse", Squidward finds a bowl of onions right under his nose on the counter. In a few shots, from behind Mr. Krabs, the onions have disappeared.
Answer: I believe this is the Season 6 episode "Spongicus".