In the episode "He's Too Sexy for His Fat", aboout halfway through, Lois gives Stewie a slice of chocolate cake. Stewie's trying to tease Chris, who's on a diet, so he digs his face in the cake and gets the frosting all over himself. Lois takes the rest of the cake and puts it on the sink behind him. When Lois isn't looking, he turns his chair around and tries to hop towards the sink to get the cake.

Meanwhile, the plot is continuing, and suddenly Peter comes in the room, skinny. Then it cuts to commercial. When it comes back on, everyone's in the exact same positions--except both the cake on the sink is gone and Stewie's face is clean.