A couple summers ago, I was over at a friend's house and we were using his pool. When we decided to get out, he went first to his room to change, and when he was done, he said I could use his room because his mom was using the bathroom.
I went in, stripped off my wet trunks and started drying myself off. After a couple minutes of this, I guess from being naked in a different place, I could feel myself getting hard. I thought about just ignoring it, but decided to go ahead and masturbate. I got in front of his mirror on his door and started stroking myself, first slow, then fast.
I was probably in there for about five minutes when I started feeling like I was going to cum. I spread my legs wide and was going to blow, when all of a sudden the door opens and his mother was standing there. She gasped and apologized, saying she thought it was her son and not me. All of this happening pushed me over the edge, and I came right in front of her with some of it landing on her bare foot. I started saying I was sorry over and over, but she said that it was OK and apologized again as she stepped back and closed the door. I cleaned myself up and went back downstairs to meet my friend.
His mother and I were both embarrassed to no end and couldn't hardly look at each other for a few days, but we got over it and I don't think she ever told him what happened.