Oops. I asked the question but didn't post mine.
It's people who lie. I don't care if it's my students lying to get out of doing something, a politician lying to get elected, or grown adults who claim not to have been on Facebook when you can see that they've posted other things during the period of time they claimed to be gone. A lie is a lie and when I catch you, it chips away at my trust in you. Not to mention that most people are very bad at it and it's very easy to tell they're spinning tales!