House Sittin' for Sandy/Smoothe Jazz at Bikini Bottom - S8-E23
Continuity mistake: When Sandy finds SpongeBob and Patrick after wrecking the tree dome, the distance between SpongeBob/Patrick and Sandy changes constantly between shots.
Karen 2.0/InSPONGEiac - S8-E28
Continuity mistake: Karen dumps a load of chum on a tray and some of the chum lands on the table. The chum that fell on the table magically disappears from shot a split second later.
The Krabby Patty That Ate Bikini Bottom/Bubble Buddy Returns - S8-E15
Continuity mistake: The giant Krabby Patty eats SpongeBob's shoe and sock in the kitchen. But by the time SpongeBob and Squidward run to the door and are stopped by Mr. Krabs, the shoe and sock have magically reappeared.
Mermaidman Begins/Plankton's Good Eye - S8-E22
Continuity mistake: When Plankton tries to open the safe, his arms come off. The arms stay on the safe. But when Mr. Krabs approaches the safe in the next shot, Plankton's arms have disappeared from the safe.
You Don't Know Sponge/Hide and Then What Happens - S8-E3
Other mistake: It is revealed in this episode that Patrick is right-handed. However in the episode "Krusty Towers", when asked to sign the Krusty Towers guest register, he is shown doing so with his left hand.
The Krabby Patty That Ate Bikini Bottom/Bubble Buddy Returns - S8-E15
Continuity mistake: At the beginning of bubble buddy, right after SpongeBob reads the letter from bubble buddy, Gary's swirl is blue and his spots are red.
Ghoul Fools - S8-E24
Continuity mistake: When Squidward is sucked into "The Void", the bag of money that he was holding vanishes in the next shot.
Audio problem: When Spongebob, is in the pile of rubble left after Squidward unraveled, he picks up his phone receiver. Then, Gary slithers away and you hear Spongebob pressing the keys on the phone, but when it cuts back to him, he is only holding the receiver.
Answer: I believe this is the Season 6 episode "Spongicus".