In the Theatre Lobby when Edward Everett Horten leaves Frederick March to go back in for the second act of the play, he blows his line but they kept it in. The line in the script is, "Well, I'm glad I ran into you, I don't want to miss the second act." What he said was, "Well I'm glad I ran into you, I don't want to miss the sar sang gat." It's so clear it's not even close. And both he and March remained in character.
Camera shadow visible on window frame as Gilda sets the table.
When Curtis (Gary Cooper) is sitting and after talking about having a white shirt, a moving shadow, probably the boom mic, can be seen to his left.
When Gilda confesses to George that she had been with Tom, after Tom asks, "Shall I leave the room?", a shadow (probably the boom mic) moves on the wall above her head throughout her speech.
Shadow of boom mic visible at train station.
Camera shadow visible right after the breakfast scene.