Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Audio problem: At the end of "Born Again Krabs", SpongeBob is talking to The Flying Dutchman on the phone. SpongeBob's dialogue isn't in sync with his mouth.
Help Wanted / Reef Blowers / Tea at the Treedome - S1-E2
Character mistake: In "Help Wanted", Stephen Hillenburg's name in the "Written By" section of the title card is incorrectly spelled as "Stephen Hilleburg."
Procrastination / I'm With Stupid - S2-E19
Other mistake: I'm With Stupid: When Patrick and his "parents" make fun of Spongebob because they believe he's stupid, he gets so frustrated that he runs through the wall, leading him right outside. This makes no sense, because they are in Patrick's house and the inside of it is underground. (00:21:35)
SpongeBob Meets the Strangler / Pranks a Lot - S3-E20
Other mistake: Pranks a Lot: When everyone is cheering on Spongebob and Patrick at the end, Sandy whistles by putting her hand through her helmet. Impossible. (00:23:30)
Factual error: In "Wormy," when Wormy's face is viewed as a close-up in butterfly form, the wrong insect is shown. As Wormy is a butterfly, there should have been circular black eyes, long antennae and a proboscis (tongue). Instead, enormous, rainbow-colored, shimmering eyes, sharp mouthparts and horn-like antennae are seen, distinctive to that of a horsefly. Furthermore, butterflies do not make buzzing sounds, which, again, is a characteristic of flies.
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.
Answer: I believe this is the Season 6 episode "Spongicus".