I've had a few golfing moments similar to Andy's, and a few other "sport"ing moments with bigger crowds and 7 figure TV audiences, but I was well in my comfort zone in all those situations.
The only place I ever lost it was when I snuck onto the practice range at Pebble Beach during the Crosby and found myself between Jerry Pate and Arnold Palmer. Pate was hitting beuatiful soft 2-irons at the 200 stake and the King was flailing away with a hundred or so admirers in tow. All I could do was hit fat irons spraying mud all over the place. Thank god none of it landed on Arnie......
To add, and get to Tom's question, I think that there is such thing as a "comfort zone" for all individuals in all circumstances. For the averge-good pro golfer, it is a normal PGA tour event, where they are programmed to grind out a good score. However, put them in the lead, and 50% fall out of their comfort zone. Put them in the lead at a major, and 90% of the remaining 50% are out of their comfort zone. Outliers come to the top from time to time (Curtis, Hamilton, Weir, maybe Johnson, etc.), but the reason Tiger wins so much is that when he is in the lead he is in HIS comfort zone. No other player has that advantage.