SpongeBob SquarePants (1999)

S1E27 I Was a Teenage Gary

When Squidward neglects to feed Gary, the poor snail falls ill, prompting the need for a doctor to administer some medicine. The doctor instructs Squidward to give Gary an injection. However, in a mix-up, Squidward mistakenly injects SpongeBob instead of Gary, resulting in SpongeBob transforming into a snail himself.