Once we saw Paul Giamatti in an airport bookstore.
We also saw Ron Jeremy, of, ahem, adult film fame in the MSP airport. My 20 year old brother was the first to spot him and started texting so fast I'm surprised his thumbs didn't start on fire. My DH discreetly pointed him out. My mother, who has difficulty realizing just how loud she's actually talking, noticed all this and asked "WHAT ARE YOU ALL BEING SO SECRET ABOUT?!" Thanks, Mom.
My DH saw Dave Matthews in Charlottesville, VA. We were eating on a patio of a pizza place (owned by Dave Matthews? I don't really remember . . . ) I didn't spot him. Since I STILL wouldn't know Dave Matthews unless he came up and introduced himself, I just kept asking if every man coming out of the pizza place was Dave Matthews.