I had it happen to me with my very first girlfriend. LOL!
Talk about always expect the unexpected. She got me good.
I'd just come home from school for the weekend (i used to have to be carted up and down the highway to a different state to go to school) She had been waiting for a while she said to me earlier that week. so when i got there she pretty much jumped me.
I got to licking her and slid my fingers in working her spot (it was right around the time i learned where the G-spot was)...she started to orgasm and, at the that time, i didn't know the climax could be as powerful as it was. She clamped down on my fingers so i backed my face away just a smidge to catch my breath and look at her face.
Right then, BAM!! right on my face. The stream ricochet'd off my cheek and into my eye and mouth. i pulled my fingers out and just blocked the stream until she calmed down.
She was so embarrassed, but we researched it and found out it wasn't that she pee'd on me.
From there, i just moved my face away whenever she'd cum.