I have one or two.
I am just learning how to trigger the third valve slide. I pulled it in without having the third valve depressed; the third slide end piece on my Bach hit the back of the Euphonium player in front of me. I imagine this one is common. Now I put a cloth elastic around the third valve slide ring and that end piece, whatever it is called.
Within the first couple of months of my comeback I had areally hard practice session in which I had over done it. Later on in the evening when brushing my teeth, I noticed something really scary and got paniced about having torn an embouchure muscle. I screamed for my wife to come take a look at what happens to my face when I smile. She said: “those are dimples you idiot; you have dimples when you smile, you never noticed before?” Actually not, but I was greatly relieved.
What is your funny trumpet story?