Jellyfishing / Plankton! - S1-E3
Continuity mistake: At the beginning of Plankton., Plankton is very small. Later in the episode, as in the rest of the show, he's bigger and not that small.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: Spongebob, controlled by Plankton, wasn't aiming directly at Squidward's house, but he eventually breaks through the wall.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: In "Plankton", when Plankton is controlling Spongebob, Spongebob goes to put his pants on, but Plankton has him walk through them. As they're walking towards the pants, the tie is red. As they're walking through the pants, the tie turns black. As they walk away from the pants, the tie is red again.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: Plankton! When SpongeBob and Plankton are talking at night Plankton gives SpongeBob a golden spatula. When Plankton says he wants a Krabby Patty for his birthday SpongeBob drops it. Then when SpongeBob walks away, the spatula is nowhere to be seen.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: In "Jellyfishing", Patrick and Spongebob slide down two ropes and burn their hands on them. The two shots after they land show them trying to cool down their hands and in the shot after that, the ropes they had just slid down on have disappeared.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: In "Jellyfishing" when Squidward comes home in the bed thingy, the king jellyfish hovers above him. At first, Squidward is facing his house. But in the next shot, he is facing the road. When he replies "Ow" he is facing his house again.
Jellyfishing / Plankton! - S1-E3
Continuity mistake: In "Jellyfishing" when SpongeBob is playing Squidward's clarinet he says "Sorry, my lips are a little dry." Then he starts licking his lips. In one shot, the clarinet disappears from SpongeBob's hands. Then it reappears in the next shot.
Naughty Nautical Neighbors / Boating School - S1-E4
Continuity mistake: In 'Boating School' when Mrs. Puff blows up she has a deep voice, but in all the other episodes her voice stays the same.
Naughty Nautical Neighbors / Boating School - S1-E4
Continuity mistake: The track changes between days.
Naughty Nautical Neighbors / Boating School - S1-E4
Continuity mistake: When Patrick runs out of Spongebob's house after Spongebob cries and says he's cheating, the lower right window beside the door is gone.
Naughty Nautical Neighbors / Boating School - S1-E4
Continuity mistake: When Squidward starts making pink bubbles with messages, he goes to his backyard. Notice he never closed the back window of his house. When he starts laughing because of the messages, suddenly the window is closed. After Patrick saves Squidward, the window is still closed, but when they go to the front of his house, the window is open.
Naughty Nautical Neighbors / Boating School - S1-E4
Continuity mistake: Boating School: When SpongeBob remembers that it's the day of the boating exam, the calendar reads "boating exam today," but then it reads "driving exam today."
Pizza Delivery / Home Sweet Pineapple - S1-E5
Continuity mistake: After Spongebob's house is gone, Squidward boo-hoos and goes into his house, then sounds happy. He starts throwing streamers and confetti out of his NOT-OPEN porthole. If the porthole was closed, they wouldn't fly out. Similar to some other ones in this show, and one in Fairly Odd Parents.
Pizza Delivery / Home Sweet Pineapple - S1-E5
Continuity mistake: In "Home Sweet Pineapple", Spongebob goes into Squidward's house to spend the night and sits in his bed with the pillow just the right size for him. But in the next shot, when Squidward climbs into bed next to him, the pillow has drastically grown and now both of them can lay on it perfectly.
Pizza Delivery / Home Sweet Pineapple - S1-E5
Continuity mistake: In "Pizza Delivery", Spongebob points out the antennae for the boat during his vehicle inspection, but in a few shots, when he's checking the tire pressure, the antennae has vanished.
Mermaid Man and Barnacle Boy / Pickles - S1-E6
Continuity mistake: At the beginning of "Mermaid Man and Barnacle Boy", when SpongeBob and Patrick are watching TV viewing Mermaid Man and Barnacle Boy show, you can see that these heroes have 5 fingers on their hands, as the real human beings. However, when SpongeBob and Patrick meet these heroes, as in the rest of the show, you can see that now, the heroes, have 4 fingers on their hands, as cartoon characters usually have.
Mermaid Man and Barnacle Boy / Pickles - S1-E6
Continuity mistake: In the episode "Pickles", there is a large fish named Bubble Bass. Although his name is Bubble Bass, the captions say his name is Bubble Bath. It is clear that Spongebob is saying "Bass" not "Bath", and his name is given as Bubble Bass in SpongeBob's Diner Dash.
Mermaid Man and Barnacle Boy / Pickles - S1-E6
Continuity mistake: Mermaid Man and Barnacle Boy: Mermaid Man wears green tights, but after SpongeBob absorbs the water ball like "some kind of evil sponge," when Mermaid Man and Barnacle Boy start doing the dog paddle, Mermaid Man's tights have vanished in this shot.
Mermaid Man and Barnacle Boy / Pickles - S1-E6
Continuity mistake: Mermaid Man and Barnacle Boy: When Barnacle Boy opens the box with the two rings inside, in the closeup the "B" ring is on the left and the "M" ring on the right, but in the wideshot Barnacle Boy removes the ring on the left (the "B") and places it on Mermaid Man's finger, then in the next shot we see it's the "M" ring, before both rings vanish.
Mermaid Man and Barnacle Boy / Pickles - S1-E6
Continuity mistake: Mermaid Man and Barnacle Boy: When SpongeBob and Patrick (dressed in their superhero costumes) exit the pineapple the front door opens at the right side (outward), but after getting on Squidward's nerves, when they retreat back inside the front door opens the opposite way at the left side (inward).
Answer: I believe this is the Season 6 episode "Spongicus".