I just found this hilarious true life story on Reddit.
This happened about a year ago when I was 17. I was home alone, and did what any teenager does when they have five minutes to themselves, I began to watch porn and masturbate.
All was well and good for ten minutes, until I heard someone coming up the stairs through my headphones. I realised it must have been my dad coming home from work and I didn't hear him open the front door. Trouble was, my door was slightly open, so I convinced myself to get up and shut it. I was also close to orgasm at this point, and it is very hard to stop that feeling once it has arisen.
Anyway, my dad is coming up the stairs and I rush from my bed to close my door, yet my climax had already begun, and I could feel the orgasm. He had reached the top of the stairs and came to my room. I couldn't shut my door in time and there he was, bursting into my room coming face to face with me, stark naked and holding my erect penis as it shoots out cum which unfortunately landed on his trousers.
I immediately slammed my door feeling absolutely horrified. I got dressed real quick and didn't speak to him for a few days. Definitely the worst possible thing I could imagine to do to one of my parents.