I interrupted my cousin jacking off once. To this day, whenever we see each other, it's become an inside joke. But it was actually really erotic, too.
My cousin Roger spent the Summer with us when I was 17. He's a year younger than I am, but he's always acted older. He's an only child, and I'm basically like a brother to him.
Anyway, so one day we had the house to ourselves, and my parents were going to be gone for awhile. I had fallen asleep in the den (now known as a media room to anyone younger than 20). I woke up, and the house was completely quiet. I went upstairs to use the bathroom, and the way our bathroom is, it had a door in the hall, and then you could walk through, and open a door to my bedroom.
Anyway, so I went to close the opposite door, which opened into my room. I reached in to grab the handle, and saw my cousin, buck naked, on my bed, with a towel behind him, just jerking away, shooting his load all over his chest. I tried to move, but he already saw me, and swore, covering himself with his hands. He's a red head, and completely covered in freckles. He was beat red, and I couldn't even see his bush against his red skin.
I shut the door, and he opened it and followed me in. "I'm sorry cuz, I thought you were downstairs, and thought I could just have a quickie." I couldn't stop laughing. He just stood there, with his hands covering his dick (not very well), still dripping cum btw, and he had cum all over his chest.
I handed him a towel, and he started cleaning himself off. I told him I do it all the time, but usually with the door closed. He was still beat red, and cleaning himself off. I noticed he was still hard, and dripping cum. He saw me looking and tried to assure me he usually came a lot more, but he'd never been interrupted. I didn't care obviously, but it was still really funny.
I told him to leave so I could use the bathroom. I flushed and washed my hands, and he came back in, and started the shower up. We swore to keep it our secret, and to this day (minus this site), I've never told anyone. He lives in Germany, so I doubt he'll ever read this, but it was erotically funny at the time.