Continuity mistake: In the Christmas morning scene after Winhert's lawyer leaves, while Nora says, "Oh, the mystery's all gone" she crosses her arms in front of her, in the next shot her arms are crossed behind her.
Continuity mistake: When Dorothy visits Nick at the hotel during the Christmas party Nick opens the door with a clear ice bucket full of ice. He goes into a room to talk with Dorothy and exits the room 2 to 3 minutes later. When he is seen leaving the room the bucket is full of water. It was too short a time for the ice to melt.
Nora Charles: Who is she?
Nick Charles: Oh, darling, I was hoping I wouldn't have to answer that.
Nora Charles: Come on.
Nick Charles: Well, Dorothy is really my daughter. You see, it was spring in Venice and I was so young, I didn't know what I was doing. We're all like that on my father's side.
Nora Charles: By the way, how is your father's side?
Nick Charles: Oh, it's much better, thanks. And yours?