Not really something that someone has done to me, but definitely the strangest thing I experienced during sex: I was coming home from a friend's house. I was in my late 20s and I went into a convenience store to get a soda and there was a woman standing in line in front of me buying a lottery ticket. She looked to be in her mid-to-late 60s. I was stoned and horny, looking for something different, so I chatted her up. She wasn't a hot GILF type though... just very plain and average for a woman of her age. Her English was ok and she was from Mexico. It was one of those occasions when you both feel that a sexual adventure is about to happen. I offered to drive her home and she eagerly accepted.
We got to her house and started getting frisky, but she told me her daughter would be home from work soon so we had to cool it. Then she had the idea that we could go out to the garage. She doubted her daughter would come out there and besides she said, we could lock the door as the garage had been converted into an apartment.
We get to the garage/apartment and started making out, then went over to the bed, ripping off our clothes. It was strange already because I had never been with anyone her age. The age difference made it feel extra kinky and I went down on her right away. She was very clean and at one point she made the comment that her vagina was "poca usada," meaning it hadn't seen much action. As an aside, her husband had moved back to Mexico many years before and she lived alone with her daughter who was a cop.
The the creepy/odd thing happened... while we were fucking in the missionary position, she told me that she was staring at the rafter above the bed (she was on her back looking up toward the ceiling) where her nephew had hanged himself the year before. So here I was fucking a 67 year old (as she told me later - we did this a few times) woman in her garage on the bed right beneath the spot where her nephew had been found hanging.
Some people might not find this strange, but it actually creeped me out. We got together 3 or 4 times and I always took her to a motel after that. Not only did I not want to run into her daughter, but I didn't like the idea of fucking in that garage again.