Jellyfishing / Plankton! - S1-E3
Character mistake: In "Plankton" when Spongebob is in Plankton's lab, he says the ingredients in a Krabby Patty. He doesn't mention the secret sauce.

Help Wanted / Reef Blowers / Tea at the Treedome - S1-E2
Character mistake: In "Help Wanted", Stephen Hillenburg's name in the "Written By" section of the title card is incorrectly spelled as "Stephen Hilleburg."

Bubblestand / Ripped Pants - S1-E1
Character mistake: Ripped Pants: During the opening credits, Paul Tibbitt's name is misspelled as "Paul Tibibitt" (it is spelled correctly the second time it is shown), and Mark O'Hare's name is misspelled as "Mark O'hare." (00:11:54)
Answer: I believe this is the Season 6 episode "Spongicus".