This was a couple years ago back in College(2014)
My friend is in the lounge area with some buddies playing cards. I wasn't playing but just hanging out, they said they needed some music so my friend asks me to go get his laptop and gave me his keys.
So I go to my friend's room looking for the laptop. As I am looking for the laptop, I hear someone messing with the door so I assumed it was my friend's roommate.
I thought it would be funny if I go into a corner and act like I was caught beating off. So I unzip my pants but still had boxers on and was in the corner faking the wanking motion. The door opens.....
It was My friend's roommate's Girlfriend.
I yelled "Oh shit!" She closes the door and leaves. So I ran to her, while trying to pull up my pants and explained I was just joking, that I wasn't really beating it. I tried shaking her hand but she looked at me in disgust.
Everytime I saw her after that it was always weird and we start laughing.