As Seen on TV / Can You Spare a Dime? - S3-E5
Continuity mistake: In "As Seen On T.V", the hat on Mr. Krabs's desk has 2 lines on the anchor instead of 1.
Sailor Mouth / Artist Unknown - S2-E15
Continuity mistake: In "Sailor Mouth" when Spongebob starts cussing in the Krusty Krab, a sailor and a pirate make a comment, but when everyone leaves the Krusty Krab, the sailor and pirate are nowhere to be seen.
Best Day Ever/The Gift of Gum - S4-E20
Character mistake: SpongeBob tries to steal the gum off his door at night. The next day, SpongeBob says that it was still Best Friends Day, so he would give Patrick his gum back. Really, it should have been the next day.
Patrick SmartPants/SquidBob TentaclePants - S4-E8
Continuity mistake: When SpongeBob and Squidward are combined, Squidward's arm is on the left and SpongeBob's arm is on the right, and that's how it is the whole time. Except in one shot, the shot of SpongeBob's head after Squidward says, "yellow freak." Look at the bottom and you'll see two SpongeBob arms, when the one on the left is supposed to be Squidward's.
20,000 Patties Under the Sea/The Battle of Bikini Bottom - S5-E17
Continuity mistake: In one scene where Spongebob is breathing in air, you can see the container to the right is yellow, but in the scene where he's dancing around the kitchen, it turns white.
Hooky / Mermaid Man and Barnacle Boy II - S1-E20
Continuity mistake: In the beginning of Hooky, there are about three boats with 3 fisherman in each. That means there would be 9, but later there are dozens.
Help Wanted / Reef Blowers / Tea at the Treedome - S1-E2
Revealing mistake: When Spongebob uses his binoculars, his teeth turn yellow for one frame.
Revealing mistake: In "Patrick! The Game," when Squidward holds out the notepad, it's obviously written on. Just when the bubble transition comes, the notepad and all the other papers on the floor clear themselves.
Your Shoe's Untied / Squid's Day Off - S2-E2
Character mistake: In "Squid's Day Off", a wide shot shows Mr. Krabs exiting the hospital, except the building says" ENEREENCY" instead of "EMERGENCY."
Help Wanted / Reef Blowers / Tea at the Treedome - S1-E2
Continuity mistake: In "Help Wanted", the last time SpongeBob says "I'm ready!", his collar turns red.
Spongebob's Last Stand - S7-E22
Character mistake: Near the end of the episode, the main characters are being introduced by SpongeBob; one of those characters is Sandy Cheeks, the name she has had for the entire show. However, SpongeBob addresses her as "Sandy Squirrel."
Krab Borg / Rock-a-Bye Bivalve - S3-E8
Continuity mistake: In Rock-a-Bye Bivalve, Spongebob's tie turns black when he is doing the chores on Patrick's second day of work, and then turns back to normal when he walks into the living room.
Mermaid Man and Barnacle Boy IV / Doing Time - S3-E1
Continuity mistake: In MMIV, there is no phone next to the door when SpongeBob shrinks the pickles. Later, Squidward calls on a phone.
Mermaid Man and Barnacle Boy IV / Doing Time - S3-E1
Continuity mistake: At the start, when Mermaid Man and Barnacle Boy run out of the Krusty Krab there are cars everywhere. When SpongeBob runs to try and get Mermaid Man's autograph inside the Krusty Krab there are no cars visible.
The Algae's Always Greener / Sponge Guard On Duty - S3-E7
Continuity mistake: When Plankton says, "Look at his daughter. She's as big as a whale!", the soda can on the table suddenly moves to the left.
Welcome to the Chum Bucket / Frankendoodle - S3-E2
Continuity mistake: In 'Welcome to the Chum Bucket', the sky is constantly changing in an abnormal way. First, it is daylight when Plankton takes Spongebob into the Chum Bucket. When Spongebob starts singing, the sun is either coming up or going down. When Mr. Krabs starts singing, look through the hole in the wall and the sky looks to be about afternoon. When Spongebob and Mr. Krabs finish their song, it's nighttime.
Chocolate With Nuts / Mermaid Man And Barnacle Boy V - S3-E11
Continuity mistake: The second time the chocolate guy chases Spongebob and Patrick, they run away and leave all their bags. But, in the next shot, they have their bags with them.
Sandy, SpongeBob and the Worm / Squid on Strike - S2-E18
Other mistake: Squid on Strike: When the destroyed Krusty Krab is first shown, the broken Krusty Krab sign has some of its letters scattered around it. There are two Ts still attached to the sign, but there is another T on the ground right next to the sign. This is impossible because there are only two Ts in "The Krusty Krab." (00:11:25)
Sandy, SpongeBob and the Worm / Squid on Strike - S2-E18
Other mistake: Squid on Strike: When Squidward looks at himself in his bathroom mirror, his buttons are on the left side of his pyjamas in the reflection. When he is shown in bed a few seconds later, the buttons are on the same side; the reflection was not reversed. (00:09:07)
The Bully / Just One Bite - S2-E17
Continuity mistake: Just One Bite: When Squidward opens the door to the Patty Vault, the door opens to the left side. In the next shot, the door is not visible anymore, and when we see it again two shots later, it is now opened to the right side. A wheel also appears on the interior side of the door, which wasn't there before. (00:09:10)
Answer: I believe this is the Season 6 episode "Spongicus".