Grandma's Kisses / Squidville - S2-E9
Deliberate mistake: Spongebob and Patrick sucks the windows off Squidward's house using leafblowers. The holes in the wall that the windows were in magically disappear.
Pre-Hibernation Week / Life of Crime - S2-E13
Continuity mistake: Sandy's bikini bottom is gone as she's bending over to paint the fence.
The Secret Box / Band Geeks - S2-E14
Continuity mistake: In Band Geeks during the Day Two scene, in the beginning you can see Pearl near the front, but in the distant shot that follows Pearl has moved further back. (00:04:20)
The Krabby Patty That Ate Bikini Bottom/Bubble Buddy Returns - S8-E15
Continuity mistake: At the beginning of Bubble Buddy Returns, right after SpongeBob reads the letter from Bubble Buddy, Gary's swirl is blue and his spots are red.
Rodeo Daze/Gary in Love - S7-E15
Plot hole: In "Rodeo Daze", there are some cows in a pen singing along with Sandy, but they are outside of the dome and the cows do not have any air helmets on.
Nasty Patty / The Idiot Box - S3-E4
Continuity mistake: In "The Idiot Box", Spongebob and Patrick are standing on the curb of the street and there is a building in the distance. Several shots later, they are standing in the same spot, but the building has disappeared.
Texas / Walking Small - S1-E18
Continuity mistake: In "Walking Small", The eel that stole Spongebob's ice cream cone throws it at his face. Different shots some show ice cream on Spongebob's nose, and some show that Spongebob's face is clean.
Your Shoe's Untied / Squid's Day Off - S2-E2
Continuity mistake: Near the start, at first you can see words on the menu board, then you can't. (00:05:50)
Bummer Vacation/Wigstruck - S4-E14
Continuity mistake: In Wig Struck, Patrick takes a bite out of Spongebob's wig and in the next shot, the wig is restored.
Continuity mistake: SpongeBob is holding a picture of Plankton and Mr. Krabs, then when he pulls Plankton and Mr. Krabs together, the picture disappears.
House Fancy/Krabby Road - S6-E1
Continuity mistake: In House Fancy, Squidward gets a bad sofa injury. In the next shot, his feet seem fine.
Continuity mistake: Throughout the whole episode of "Missing Identity", SpongeBob's shirt is backwards. This is seen with his sleeves being supposedly behind him. The sleeves are missing from the front, which is normal, but throughout the episode, pay attention to behind SpongeBob. His sleeves are supposed to be behind him, but are missing altogether.
Sandy's Rocket / Squeaky Boots - S1-E8
Audio problem: In "Sandy's Rocket", when Sandy says "Aliens? Is that what this is about?" her mouth stops moving halfway into her sentence.
The Krabby Patty That Ate Bikini Bottom/Bubble Buddy Returns - S8-E15
Other mistake: Mr. Krabs pours Sandy's growth serum on a patty. It grows, and Spongebob scrapes some off to construct individual patties from the larger one. The larger one then continues to grow, but the individual ones do not, even though they are all the same meat.
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.
No Weenies Allowed / Squilliam Returns - S3-E6
Continuity mistake: At the beginning of "Squilliam Returns," Squilliam is walking to Squidward from the left of the garbage pails. When Squidward throws his hat out, Squilliam is to the right of the garbage pails.
No Weenies Allowed / Squilliam Returns - S3-E6
Continuity mistake: In "No Weenies Allowed," when Spongebob gets punched by Patrick, he doesn't have a black eye. When the shot zooms to Spongebob's face, he has a black eye.
MuscleBob BuffPants / Squidward, the Unfriendly Ghost - S1-E11
Plot hole: In MuscleBob BuffPants, when SpongeBob first shows off his new inflatable muscles to everyone on the beach, he had the strength to punch two muscular fish into the air with them. A minute later, he could not even lift a glass of juice with the inflatable muscles. (00:04:25 - 00:05:20)
Other mistake: Always in the older episodes (and sometimes in the newer ones), when a fish in the background takes his/her second bite of a sandwich, the sandwich still looks like the first mark. It does this on every bite after the first.
Answer: I believe this is the Season 6 episode "Spongicus".