I forgot my clarinet on the night of the state game my first year in marching band. I marched with my hands fisted one over the other like when you practice without instruments while you're blocking. It was televised
My son played saxophone in jazz band in middle school and forgot his mouthpiece at a jazz festival. Trouble was, he was really good. The director made him borrow another kid's mouthpiece to play.
Aside from how incredibly gross that was, I often wonder how it made the other kid feel.
"You forgot your mouthpiece? Well, we can't have you not play. Borrow a mouthpiece from that kid. He sucks anyway."
In middle school I played the (French) horn, and one time I lost my mouthpiece right before a concert. I spent the entire thing just pretending to play my mouthpieceless instrument.
I think my teacher knew not to do a song with a horn solo; none of us actually bothered to learn the notes to the songs. The concert probably sounded marginally better without me.
A kid forgot his instrument but showed up to a performance in middle school and the teacher told him to "sit in his seat and make the gestures. No one will see you anyway."
I was too busy with my own music to watch but I know he did it. I bet it was amazing.