When I was in Denver I had a lady run up to the precheck line without a boarding pass and try to get through. Apparently she thought it meant you could go through this line before you checked in (ie precheck).

I have to say that this was the most patient TSA lady I have ever seen. Actual quote:
"Maam, I can't let you through here without a boarding pass"
"But you are going to make me laaaaaaatttteeeeeee"