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.
Bubblestand / Ripped Pants - S1-E1
Revealing mistake: In "Ripped Pants", Spongebob plays the guitar and chord changes are heard, but his fingers on the neck of the guitar do not move.
Help Wanted / Reef Blowers / Tea at the Treedome - S1-E2
Revealing mistake: When Spongebob uses his binoculars, his teeth turn yellow for one frame.
Help Wanted / Reef Blowers / Tea at the Treedome - S1-E2
Revealing mistake: In the shot where Spongebob says, "That little squirrel is in trouble, " half of his belt is missing.
Culture Shock / F.U.N. - S1-E10
Revealing mistake: In "Culture Shock" when Spongebob takes the green fish's order, he has a pencil tucked in his ear, but he doesn't have an ear, so it's just floating in thin air. (00:01:55)
Revealing mistake: After SpongeBob disguises himself as Patrick's rock and right before he sneezes his uvula clips through his bottom lip.
Revealing mistake: At the beginning of Rock Bottom, when the bus pulls up in front of Patrick and SpongeBob, right before Patrick steps up onto the bus, SpongeBob's right arm is missing and it suddenly appears as he starts moving.
Revealing mistake: Near the end of "Rock Bottom", when SpongeBob arrives back to Bikini Bottom, SpongeBob's pineapple is animated differently from the other buildings. It looks like it was cheaply tacked on in post production.
Big Pink Loser / Bubble Buddy - S2-E3
Revealing mistake: Big Pink Loser: When Patrick is answering phone calls, the third time he picks up and says, "No, this is Patrick", the anchor symbol on his hat briefly vanishes. (00:04:36)
Big Pink Loser / Bubble Buddy - S2-E3
Revealing mistake: Big Pink Loser: When Patrick says, "Why can't I do anything right?", and starts hitting his broom on the floor, the drinks that are supposed to be on the customers' tables are instead layered behind them. (00:05:18)
Mermaid Man and Barnacle Boy III / Squirrel Jokes - S2-E4
Revealing mistake: When we see Billy get told about the stupid squirrel, he has a sleeve going through his arm. The moment he starts talking, the sleeve disappears and his arm extends over his shirt. Around the end of the shot when he's being pulled away, the sleeve returns.
Dying For Pie / Imitation Krabs - S2-E6
Revealing mistake: In the episode "Dying For Pie", you can see Mr. Krabs' legs disappear when Squidward says, "But you can't pay me enough to act brotherly on that guy!" Also, Squidward is standing on 2 tentacles instead of 4.
Revealing mistake: At the end of "Wormy", Patrick's eyes are missing.
The Secret Box / Band Geeks - S2-E14
Revealing mistake: When Squidward asks who has played instruments before and Plankton responds, look at Plankton's triangle. The bottom disappears for a split-second.
As Seen on TV / Can You Spare a Dime? - S3-E5
Revealing mistake: When SpongeBob is mopping the floor, two kids walk into the background and put mustard and ketchup on their patties. Nothing is coming out of the condiment tap.
Krab Borg / Rock-a-Bye Bivalve - S3-E8
Revealing mistake: In the shot that shows the outside of the Krusty Krab (in Krab Borg) they reused the drawing of right before the customers ran out of the building because you can see many customers crowded at the door. This shot lasts about 2 seconds but it's not hard to spot. (00:08:05)
Chocolate With Nuts / Mermaid Man And Barnacle Boy V - S3-E11
Revealing mistake: In one scene with Spongebob, Patrick, an old lady, and her almost dead mom, in one part Spongebob has no holes on his face. There is also a light-colored spot near Patrick's mouth.
Club SpongeBob / My Pretty Seahorse - S3-E12
Revealing mistake: In Club SpongeBob, when SpongeBob, Patrick and Squidward are in the forest, Squidward asks the magic conch shell if he can have something to eat. Straight after he asks "Can't you say anything else but 'No'?", it has a close-up of the shell. But Squidward's arm appears to be yellow, like SpongeBob's. This error was later corrected in 2019 when the episode was remastered for digital releases.
Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Revealing mistake: I Had An Accident: Almost every time the actor in the gorilla suit is on screen, he is transparent, with the background showing through him. (00:20:48)
Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Revealing mistake: I Had An Accident: When the gorilla is beating up Patrick and Sandy in the bag, in some shots, there are faint lines on the screen due to the green screen being cropped incorrectly. In addition, when the gorilla smashes the chair onto the bag, some pieces of the chair cross one of the lines and disappear. (00:21:02)
Answer: I believe this is the Season 6 episode "Spongicus".