The Kipper and the Corpse (a.k.a. Death) - S2-E4
Revealing mistake: When the characters are in Mr Lehman's room discussing his death, the backdrop outside the window has clear wrinkles and fold marks in it even though it's supposed to look like the outside.
The Germans - S1-E6
Continuity mistake: When Basil is being pursued by the doctor, the left antler on the moose's head on the wall is broken. When the head lands on Manuel, the antler is not broken or limp any longer, and when the Major wraps the head on the snout, the antler breaks and goes limp again.
The Germans - S1-E6
Character mistake: When the first German guest enters reception, he asks Polly, in German, if she would tell him what time lunch is served. She replies in German "at 9am, in five minutes." Which would be a strange time to be serving lunch! Actually her pronunciation of "9" sounds more like the German word for "no", so could be mistaken for "at no time. In 5 minutes."
Suggested correction: He asks what time lunch is being served. Lunch would not be served at 9am, it would be served around midday. Polly's response is "In no time. In 5 minutes" is absolutely correct. She does say "nein", not "neun" but it is not a mistake.