Bill Murray
Literally ran into him on the street in NY
Walking into a stiff winter wind on 57th street in NYC, I had my head down to avoid the cold, and ran into someone. I took a step back, looked at the person, and my mind registered that it was Bill Murray, but did not process it fully. I said "Oh, hi Bill!", and I walked on. Two steps later I realized what I had done, and without turning said out loud "Oh hi Bill?!!?" and turned around to see him finish getting into his limo to drive away.