Clean Lovin' / Unbalanced Load - S1-E13
Other mistake: In the credits, Tom Kenny is credited for voicing Heffer, but Heffer does not make an appearance in either episode. (00:21:54)
Suggested correction: I think it's kind of nebulous whether or not this should count as a mistake. From what I gather, it's usually contractually stipulated (and sometimes enforced by union rule) that if you play a lead/regular character on a series, you're supposed to always appear in the credits, even if you miss a random episode here and there. As a result, this is actually relatively common. I almost feel like this would be better as a trivia entry. Or if these types of "meta"/fourth wall mistakes (credit errors, DVD box errors, etc.) had their own listing.