Online, I have played a lot of pros on Full Tilt. Mostly in tourneys. My favorite memory was playing Jim McManus in a sat, one winner, it got to HU and I won.
Live, I was in a tourney at Foxwoods a couple of years ago and for some reason, there were a ton of pros in it despite it not being a big buyin. I recognized a lot of faces without necessarily being able to put names to them, had definitely seen them on TV. I was fairly new to poker then and didn't really want such competition.. I'd lasted to the 3rd break and my table broke up, was moved to another table. I was setting down my chips when I peeked at my cards, liked them OK and said call.
Unfortunately, another player had already raised it up and I of course had to stay in. I said who? and at the other end of the table, with a massive stack of chips, Chris Ferguson grinned at me. (Yeah, recognized him.) Oops. (I learned a little something from that, let me tell you: know what the action is ahead of you!) I had outs but they didn't get there. Out before I even set up my chippies. Couldn't help but laugh.