They're so easy to tell and lots of the time they're easy to maintain and hide. Really though, they just destroy trust so quickly...
I have a fantastic memory, I can recall random trivial facts or stories or anecdotes from years and years ago (unfortunately this doesn't stop me from forgetting my purse everywhere). So it's also really easy for me to catch people in their lies, I don't set out to purposefully do this. It just sort of happens because I'll hear them lie to me and I'll instantly remember something they said a few days or weeks earlier that completely contradicts that. While I usually don't mention them or make a big deal out of them when they happen, when that person has specifically lied to me about something it is really difficult for me to trust them again in that area... I hate it when people lie to me because it usually seems so pointless. I always end up being more upset that they lied to me about something than I would have been about whatever it was they were trying to hide.
Trust takes years to build and only a second to destroy... It's a cliche for a reason. It is really true!