High school, final year.
We're all eating lunch in the playground, when someone spits at someone else, then someone else spits back, and suddenly it's erupted into an all-out spitting war.
My mate Stewart hoiks up a really solid one from the depths of his lungs and lobs it into the air. I'm laughing or yelling or something, can't remember exactly what, but my mouth is open. Stewart's golly lands in my mouth, right at the back of my tongue, I feel it bounce & hit the back of my throat. The swallowing reflex kicks in and down it goes. To this day I still gag if I think about it hard enough.