How did Karen actually reach the conclusion she was being cheated on? All she found was the necklace, which she then assumed wasn't for her on Christmas Eve, when it could've been stashed elsewhere as a surprise (I know we knew it wasn't, but how would she have known?) Was it supposed to be a comment on her intuition or did they cut a scene here?
shareI always thought there must have been hints elsewhere that we didn't get to see, because at the Christmas party when she's watching him dance with the other woman, she already seems to slightly suspect. Especially since she says "be careful there" after she mentions how pretty the other woman is.
sharepresumably as all the presents were under the tree with people's names on, she knew which ones were for her. And that was the only one the right shape to be the necklace box. So she assumed (rightly) that she wasn't getting the necklace.
shareThere is already a thread on this question:
http://www.imdb.com/title/tt0314331/board/thread/251208702
Simple. The note apologizing for being so grumpy is the note he would put on the BEST gift.
shareYes her intuition, isn't that obvious?
Who else would the necklace be for? It's so obvious!
If you read the trivia section, it answers your question.
He had a long torrid affair. They stayed married but it was never the same.