Season 7 Episode 18 : To SquarePants or Not to SquarePants
After his square pants shrink in the dryer, SpongeBob starts wearing round pants, causing people to not recognize him.
Previously: Season 7 Episode 17 : Square Roots: The Story of SpongeBob SquarePants
Next: Season 7 Epsiode 19 : Chum Caverns