For me, it's a tie, both with the same girl who I dated in HS and my first year in college. The one time started off not too strange (but became really strange), at her house in the basement with her family upstairs. We were lying on the carpet, watching TV, having a nice, long, liesurely fuck, spoon-style, with my pants pulled down to my kness and her skirt pulled up. Suddenly, we heard her mom start coming downstairs and I quickly pulled a blanket over us--it was winter and must have looked pretty natural. Her mom just came down to ask me if I wanted to have dinner with them, but then got caught up watching the movie on the TV with us. I continued to slowly fuck her daughter, no more than 6 feet away from her, for about 30 minutes, until after the movie ended and she left us alone again. My girlfiriend was torn between wanting to beat the shit out of me for not pulling out and continuing to fuck her, and laughing hysterically about the whole thing.
The other strange place was in our college dormatory. One evening we were in the basement looking to see if there laundry room wasn't too full. After finding it vacant, we were on our way to go and get our laundry when I accidentally bumped into the door to the dorm's cafeteria kitchen. Somebody had forgotten to lock it and we snuck in to explore a little. Always a little frisky, I occaisionaly grabbed my girlfriend, eventually leading into some making out in the dark kitchen. Passion overwhelmed us and we wound up fucking like rabbits ontop of the stainless steel food prep table. She was always very wet, and with me shooting a good load, we left quite a mess on the countertop. At each meal the next day, we were both continually cracking up about both what we had done and whether or not they wiped down the counter surface before making the food. Tereribly unsanitary, but hugely funny to a pair of 18 year olds.