To see this slip up, you must be watching the "Invader Zim" episode "NanoZim" (It is episode 2B). Watch when Gaz and Dib are talking at the Skool. After Dib says "Yeah, sick with fear!" he takes a bite of potatoes. Later, when Zim starts to tell Dib how he got inside his body, it shows the exact same clip, but Zim is in the peas, and Dib takes a bite of the peas.