"It's complicated." This one is typically used in response to the question "Who was that?" after your significant other has been talking to someone you don't recognize. In that context, it's ALWAYS a lie. Personally, I think women use it more often than men, but I've heard of both scenarios among people I know. The truth, of course, is that the person's relationship to your s.o. isn't complicated at all. The liar is just saying this to spare your feelings.