The Belchies - S2-E1
Continuity mistake: After they escape the tunnels, Tina's shirt instantly brightens in colour after she comes to a standstill.
Continuity mistake: The pillow disappears and reappears between shots while Gene is on the bed. Interestingly, the wrinkles his feet make in the pillow can still be seen floating in midair in the shot where the pillow itself is missing.
Answer: S04e15, "The Kids Rob a Train."
Bishop73