Monkeyman! / Buses, Bikes and Subways - S5-E1
Other mistake: Buses, Bikes and Subways: When Helga and Harold are running to catch the bus, as Harold says, "Ow, quit pulling my arm," Helga is frozen at the right side of the screen for a second before she starts moving. Also, her mouth doesn't move when she says, "The bus is leaving, you dumb oaf." (00:13:59 - 00:14:53)