Sandy, SpongeBob and the Worm / Squid on Strike - S2-E18
Corrected entry: In "Squid on Strike", the narrator at the end shows "one eternity later", and we see SpongeBob and Squidward as skeletons. SpongeBob has no bones, he said that in "Squirrel Jokes". And squids have no bones either.
Sandy, SpongeBob and the Worm / Squid on Strike - S2-E18
Corrected entry: In "Sandy, Spongebob, and the Worm," when the town is in the Krusty Krab talking about how to protect themselves from the Alaskan Bull Worm, the pink fish with the red bikini is in the crowd twice and there's another fish that's in there three or four times.
Correction: This happens in animated shows plenty of times. These characters are 'stock' characters, and aren't at all useful to the plot, and are therefore used more often than characters that actually have bigger parts. This is not a mistake - however, if someone such as Pearl or Larry were to be 'duplicated', that would be a mistake, because they actually have recurring and speaking parts.
Correction: The fact that they have skeletons is part of the joke, and therefore doesn't have to be entirely factual.
shorty30490