House Fancy/Krabby Road - S6-E1
Continuity mistake: Plankton's size changes drastically throughout Krabby Road. Plankton can be seen as reaching up to Spongebob's waist at one point. When he's clutching to the neck of a guitar, the amount of frets that go into his height change between one, two, three, and five, depending on the shot. (00:02:10)
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.
Penny Foolish/Nautical Novice - S6-E2
Continuity mistake: In "Penny Foolish" around the beginning, SpongeBob does a lot of stunts before he picks up the penny. Then when Mr. Krabs flashbacks to what SpongeBob did, the buildings in the background change.
Penny Foolish/Nautical Novice - S6-E2
Continuity mistake: In "Penny Foolish" when Mr. Krabs is driving at night, there are a bunch of buildings surrounding him. Then when the shot cuts to the steering wheel and back to Mr. Krabs the buildings disappear.
Spongicus/Suction Cup Symphony - S6-E3
Continuity mistake: In "Spongicus" when Mr. Krabs and SpongeBob are outside looking at the "Chum Coliseum" and Mr. Krabs says "it's very suspicious" the tables inside the Krusty Krab are missing.
Revealing mistake: In "Not Normal" when SpongeBob says "Hmm. This video tape should help," his left arm is missing. (00:01:25)
Giant Squidward/No Nose Knows - S6-E7
Continuity mistake: In "Giant Squidward", Squidward goes to spray his wilting kelp with fertilizer and sets down a watering can right next to him. When the kelp goes back to normal in the next shot, the watering can disappears and is not seen again.
Giant Squidward/No Nose Knows - S6-E7
Continuity mistake: In "No Nose Knows", Patrick is trying out his new nose. He passes through a cloud of perfume sprayed by a fish in front of the perfume shop. In the next shot, the fish and the perfume shop are gone.
Boating Buddies/The Krabby Kronicle - S6-E9
Other mistake: In "Krabby Kronicle," when the two scientists exit Sandy's air dome, there are bubbles trailing behind them.
Boating Buddies/The Krabby Kronicle - S6-E9
Continuity mistake: In "Krabby Kronicle," when the female fish takes a copy of The Bottom Feeder, the pile of newspapers stays the same.
Boating Buddies/The Krabby Kronicle - S6-E9
Continuity mistake: In "The Krabby Kronicle", Spongebob takes a picture while holding the camera diagonally, but when the picture is printed, it is horizontal.
Boating Buddies/The Krabby Kronicle - S6-E9
Continuity mistake: In "The Krabby Kronicle", a smaller fish punches Larry the lobster in the gut. When they finish their conversation, they are standing in the sand. When they walk away in the next shot, they are in the road.
Boating Buddies/The Krabby Kronicle - S6-E9
Continuity mistake: In "The Krabby Kronicle", Mr. Krabs hands Spongebob his spatula next to the printing press. In the next shot, he is not holding the spatula, nor is it on the floor.
Krusty Krushers/The Card - S6-E13
Audio problem: In "The Card", when Patrick shows SpongeBob the four special talking cards, watch carefully. You hear four punches landing on Barnacle Boy, but see only three fists come in from the right side.
Shuffleboarding/Professor Squidward - S6-E17
Other mistake: Shuffleboarding: When the jail explodes and all the fish fall to the ground, one fish hits the ground and then disappears mid-shot. (00:10:45)
Gullible Pants/Overbooked - S6-E19
Other mistake: In the episode where SpongeBob has to do 3 things in 1 night, near the end, they all get frustrated for not doing their tasks. Look at Mr. Krabs. He runs over to SpongeBob and behind him is bubbles. Impossible, because in Sandy's dome is air.
Gullible Pants/Overbooked - S6-E19
Continuity mistake: In "Gullible Pants" when Mr. Krabs is about to exit the Krusty Krab, he looks at his watch, which disappears in the next shot.
Gullible Pants/Overbooked - S6-E19
Other mistake: In Overbooked, when Patrick tells Sponge-Bob about his birthday, Sponge-bob put a Krabby patty in his pants. In the next scene when you see Sponge-Bob, the patty is gone.
Chum Bucket Supreme/Single Cell Anniversary - S6-E22
Other mistake: In Chum Bucket Supreme, when Plankton is changing his sign for his restaurant, the door to the Chum Bucket is missing. (00:01:10)
Chum Bucket Supreme/Single Cell Anniversary - S6-E22
Continuity mistake: In "Single Cell Anniversary", Spongebob places a megaphone on the ground. In a few shots, the megaphone has disappeared by itself.
Answer: I believe this is the Season 6 episode "Spongicus".