During, or in anticipation?
For anticipation, probably the 2000 US Open golf tournament, although that turned into a snoozer because Tiger Woods just ran away with it. Maybe a college basketball game in 1998 when North Carolina played at Cal and Cal won in an upset. It was during the 1998-1999 NBA lockout where there was a lot more interest where tickets were kind of hard to come by.
During, I remember a college basketball game that might not have been one for the ages, but it was near sold out, went two overtimes, and was won by a point via a floater in the lane with about 2 seconds left. I also went to the game where Klay Thompson scored a record point total in a quarter. Both of these games I attended on a whim. I decided to go to the college game after work and there were tickets available (actually for anyone to be in the general admission student section). The latter I was actually there to buy Disney on Ice tickets where the box office was open and I could save on Ticketmaster convenience fees. But there were single tickets available (even though technically sold out) although I bought one from someone approaching me with a group ticket where someone couldn't make it. I paid a little bit less and got a better seat.