Stopover in a Quiet Town - S5-E30
Continuity mistake: Bob's cigarette sets the artificial grass on fire, and he rapidly stamps it out. When he finishes extinguishing the flames, the grass is scorched, but intact. He doesn't touch it again, but one shot later, the burned grass and all its ashes have vanished and a huge hole has appeared, revealing the metal mesh underneath. (00:13:50)