Continuity mistake: Hogan gives a birthday party when the prisoners capture a German plane. When it is taking off, it is an English Lancaster bomber with a twin tail. As it is flying overhead, it is an American B-17 with a single tail. Finally as it is shown flying, it is a German FW-200 maritime bomber.
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.Movie Nut
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.