OK this doesn't happen that often any more probably because I am older and don't really care as much but when I was younger and got really nervous, I would believe that I had - this is kind of gross - a booger hanging out of my nose. I would start to pinch my nose or cover it up with my hand because I just knew there was something hanging out. This would happen to me in a high pressure situation, like a job interview or just talking to someone new. Really annoying how the mind can plays tricks on you.