Erie, PA in 1985. The stands were really low and really close as there's no track. We finish the OTL and put our horns, and I was real close to the sideline. The crowd just starts yelling and jumping, and there was this older lady who I focused on because she was right in front of me. She didn't stand up, but she mouthed something like "Oh my God!", covered her face with her hands, and burst into tears. One of those seminal moments in my marching career.
That, and the US Open prelims in 85 again, where, before we went on, our staff told us "This crowd goes crazy if you seed the applause. So we're not going to sit together in the stands and act like regular audience members and clap at everything." Well, it worked. The audience screamed and clapped at spots we'd never heard applause before. They went crazy! And that was the first time we beat SCV that year.