Continuity mistake: When we see the "Flabby Patty" for the first time, there are 8 fish in line. When we see it the second time, there is only 5.
New Fish in Town/Big Sister Sam - S8-E10
Audio problem: In "New Fish in Town", Squidward pops out of his door and says "the bassoon." When he says it, the words are heard, but his mouth movements are delayed.
Continuity mistake: When SpongeBob scrambles into the Krusty Krab to tell Mr. Krabs the "good news", the doors swing wildly inward. When the two return outside and see the Flabby Patty, the doors have varied from being open inward, to being open outward.
Buried in Time/Shellback Shenanigans - S8-E5
Continuity mistake: The authentic plate that SpongeBob accidentally drops in the time capsule and breaks repairs itself when it is seen inside the capsule after being buried.
Karen 2.0/InSPONGEiac - S8-E28
Continuity mistake: Squidward's house is missing in the shot that shows SpongeBob running from his house to Patrick's house.
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.
Continuity mistake: SpongeBob is holding a picture of Plankton and Mr. Krabs, then when he pulls Plankton and Mr. Krabs together, the picture disappears.
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.
Squidward's School for Grown Ups - S8-E18
Continuity mistake: Spongebob uses a razor to try and shave off Patrick's beard, which falls onto the ground. In two shots, the razor disappears.
Karen 2.0/InSPONGEiac - S8-E28
Continuity mistake: In "InSPONGEiac", Spongebob drinks from a glass and drops it on the floor. In two shots, the glass has disappeared.
Revealing mistake: When Spongebob and Mr. Krabs are rolling in the pile of money, dollar bills are flying into the air. After crossing an invisible line, the dollar bills disappear in mid-air. (00:11:35)
Barnacle Face/Pet Sitter Pat - S8-E21
Continuity mistake: In "Barnacle Face", Spongebob is holding a glue bottle at the top of a ladder. When he climbs down, the glue bottle is not in his hand or on the ladder.
Barnacle Face/Pet Sitter Pat - S8-E21
Continuity mistake: In "Barnacle Face", Pearl throws a load of bars of soap at Mr. Krabs as he exits the room, leaving them scattered on the floor. In a few shots, they have disappeared.
Mermaidman Begins/Plankton's Good Eye - S8-E22
Continuity mistake: In "Mermaidman Begins", Mermaidman and Barnacleboy are running across a dock, about to jump into the sea. Next to them, on the right, there is a pelican sitting on a pillar. In the next shot at a different angle, the pelican is gone.
Mermaidman Begins/Plankton's Good Eye - S8-E22
Continuity mistake: In "Mermaidman Begins", the first time it shows Ernie microwaving the popcorn, there are 21 buttons on the microwave. In a later shot, there are now 24.
Mermaidman Begins/Plankton's Good Eye - S8-E22
Continuity mistake: In "Mermaidman Begins", the scientist experimenting on the ants knocks over the jar containing five ants. In the next shot, there are eight ants on the floor.
Mermaidman Begins/Plankton's Good Eye - S8-E22
Continuity mistake: In "Mermaidman Begins", the camera goes up one story to show a scientist. On the poster next to him, it shows various molecules and particles. In the next shot, the word "science" appears on the poster.
Karate Star - S8-E2
Continuity mistake: Spongebob is mopping the floor when a customer walks in the tell him about Patrick chopping everything. Two shots later, the mop and the water on the floor disappear.
Continuity mistake: Spongebob is looking up at the roots that are bouncing above him. In the first shot, there are only three above him. In the next shot, it shows more roots, with Spongebob standing under four.
Tunnel of Glove/Love That Squid - S8-E14
Continuity mistake: In "Tunnel Of Glove" the amount of police tape on the entrance of the tunnel of glove changes. Some shots there are three pieces of tape, others there are four.
Answer: I believe this is the Season 6 episode "Spongicus".