Falcons @ Bills - 2005. I brought my girlfriend (at the time) to attend the game with me. We were seated in my season tickets in the Rockpile. At one point, second half, I believe, a fight got started a couple of sections to our right (almost mid endzone, right below the scoreboard). It didn't take long for things to escalate, with more and more people fighting - I was obviously trying to watch the game and keep an eye on her, so I didn't really see it unfold. It took a while for security to get things under control (probably 15-20 minutes, real time) and when it was all over, a person in a Falcons jersey was taken out on a gurney, bleeding. Rumor has it that the person was stabbed.
Regardless, I wasn't able to convince her to attend any games with me again. Thank you rowdy Bills & Falcons fans.