Other mistake: In this episode, the German spy Hogan conducts around the Underground tunnels sees things (i.e., steam room, weapon manufacturing line, barber shop, etc.) that are never seen in the series again.
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Other mistake: Klink has on his monocle, as usual. If you look closely, the monocle is a single lens, with no frame. From Episode 2 onward, the monocle has a frame and a gallery. Also, Klink wears it at an angle, not unlike the Penguin in the original Batman series, who also wore a monocle without a frame.