He's Too Sexy for His Fat - S2-E17
Continuity mistake: In the Griffin home establishing shot just before Peter enters the house with new muscles, it shows the walkway to the front door and mailbox is clear of any obstacles. Yet a minute later, Lois pushes Stewie out the front door on a wagon, and he rolls down the walkway, crashing into some trashcans that have suddenly appeared behind the mailbox. (00:14:35 - 00:15:20)