Bummer Vacation/Wigstruck - S4-E14
Other mistake: Bummer Vacation: When Spongebob says, "You're right, Gary. It is unhealthy to think of the Krusty Krab while I'm on vacation," his legs are missing. (00:05:55)
The Lost Mattress/Krabs vs. Plankton - S4-E2
Continuity mistake: Krabs vs. Plankton: When Krabs meets with the attorney, Bottomfeeder, both the exterior and interior door knobs on his office door appear and disappear in various shots, even consecutive shots.
Have You Seen This Snail? - S4-E3
Continuity mistake: Have You Seen This Snail: After the customer asks for a refund it cuts to SpongeBob crying in the kitchen, and when Krabs walks in the door opens at the left side with no knob, but when Krabs tells SpongeBob that a problem can be solved with a little hard work, that door now opens at the right side with a door knob.
Driven to Tears/Rule of Dumb - S4-E17
Continuity mistake: In 'Rule of Dumb' Patrick's house returned to its former self in the outside shot just before Squidward's house was moved.
The Thing/Hocus Pocus - S4-E16
Continuity mistake: In 'The Thing' the house of Squidward's door first opened outwards, then later inwards. It also changes color.
Selling Out/Funny Pants - S4-E5
Continuity mistake: In "Selling Out", Mr. Krabs hears the footsteps of many customers approaching and the shot shows the entrance to the Krusty Krab from the inside, with no one in sight. When Mr. Krabs tells the guys that they're coming, it shows Squidward in a few shots mopping right in front of the doors, even though he wasn't seen there previously.
Selling Out/Funny Pants - S4-E5
Audio problem: In "Selling Out", Mr. Krabs explains to Spongebob that he's retiring. When he says "bowls of fruit", his mouth doesn't match the movements.
Best Day Ever/The Gift of Gum - S4-E20
Continuity mistake: When SpongeBob tries to steal the gum at night, he's dressed as a bandit, but then the next day when Sandy finds him, he's wearing his normal clothing. He was stuck in the gum all night so there's no way he would have changed.
Continuity mistake: In "Once Bitten", there is a keyhole on one of the doors to the Krusty Krab. Several shots later, the keyhole disappears.
Krusty Towers/Mrs. Puff, You're Fired - S4-E9
Continuity mistake: When the fish is at the front desk of the Krusty Towers, there is a silver bell on the desk. When Patrick arrives, there is no bell. Then Mr. Krabs puts Squidward in his place, and the bell is back, but has moved to the opposite side of the desk.
Ghost Host/Chimps Ahoy - S4-E10
Continuity mistake: In "Ghost Host", Spongebob rips off a bit of sponge and drops it on the ground. Two shots later, his head is back to normal and the sponge on the floor is gone.
Ghost Host/Chimps Ahoy - S4-E10
Continuity mistake: In "Ghost Host", the Flying Dutchman is falling down the side of a cliff. When he lands on the ground in the next shot, the cliff is gone.
Ghost Host/Chimps Ahoy - S4-E10
Continuity mistake: In "Chimps Ahoy", a robot is lying down on a log. The robot gets up and peels the banana one of the chimps was holding. In the next shot, the log has completely disappeared.
Ghost Host/Chimps Ahoy - S4-E10
Continuity mistake: In "Chimps Ahoy", Spongebob removes a helmet he was wearing and holds it in his hands out of frame. In the next shot, the helmet is missing and not on the ground.
Squidtastic Voyage/That's No Lady - S4-E15
Continuity mistake: In "That's No Lady", Patrick goes up to the man with a briefcase holding a pad of paper and a pencil. In two shots, the pad and pencil Patrick was holding disappears.
Squidtastic Voyage/That's No Lady - S4-E15
Audio problem: In "That's No Lady", Mr. Krabs throws a megaphone behind him, but there is no sound of it clattering after him throwing it.
Squidtastic Voyage/That's No Lady - S4-E15
Continuity mistake: In "That's No Lady", there is a place card on the table Mr. Krabs is sitting at. When he puts a sheet over the table, the spot where the card was, doesn't bulge.
Squidtastic Voyage/That's No Lady - S4-E15
Continuity mistake: In "That's No Lady", Patrick has some type of glasses, which he takes off his face and holds in his hand. In the next shot, they disappear.
Squidtastic Voyage/That's No Lady - S4-E15
Continuity mistake: In "That's No Lady", Spongebob has a tray with two lemonades and gives one to Patrick. In two shots, the tray and the two glasses of lemonade disappear.
Squidtastic Voyage/That's No Lady - S4-E15
Continuity mistake: In "Squidtastic Voyage", Squidward drinks many bottles of soda to try and get Spongebob and Patrick out of his stomach. Two shots later, these bottles all disappear without anyone touching them.
Answer: I believe this is the Season 6 episode "Spongicus".