Revealing mistake: At the end of "Wormy", Patrick's eyes are missing.
Continuity mistake: When Tom and Fred are outside the Krusty Krab, Tom is light green, but later in this episode he is dark green. For more info on Tom: http://en.spongepedia.bimserver.com/index.php?title=Tom For info on Fred: http://en.spongepedia.bimserver.com/index.php?title=Fred.
Continuity mistake: When Patrick is dressed as food for the "monster" in "Wormy," when the butterfly is on P,atrick's butt he spits out the apple in his mouth and it floats to the top of the water helmet. But when the butterfly is on his helmet, the apple is gone.
Other mistake: At the very end, Sandy puts Wormy inside of a glass jar. Not only does she break the bubble surrounding it, there's no way for water not to enter the jar as she holds it sideways. Wormy should be killed.
Factual error: In "Wormy," when Wormy's face is viewed as a close-up in butterfly form, the wrong insect is shown. As Wormy is a butterfly, there should have been circular black eyes, long antennae and a proboscis (tongue). Instead, enormous, rainbow-colored, shimmering eyes, sharp mouthparts and horn-like antennae are seen, distinctive to that of a horsefly. Furthermore, butterflies do not make buzzing sounds, which, again, is a characteristic of flies.
Answer: I believe this is the Season 6 episode "Spongicus".