I'm old and unlucky enough to have had many bad sexual experiences, for better or for worse. Unfortunately, by modern standards, I've definitely had some questionable experiences where I was taken advantage of (i.e. intoxicated, emotionally vulnerable, etc) and which would likely fall into the non-consensual category these days, so I'll leave those out, since they were the worst by far.
I've had plenty of bad experiences that were totally on me, so for the sake of my own ego, I'm going to disqualify those.
The worse consensual experience I can recall at the moment was awkward and a bit weird. The guy was persistent and braggadocios while pursuing me. He was nice looking and we got along fine online and in phone calls, so when the conversation came up about meeting up for nothing serious, I was okay with that.
We hung out awhile before getting intimate and he was nice enough, nothing seemed off. It's almost a stretch to call what we did together "sex", though. He wasn't able to get an erection. Not a problem honestly, there are still plenty of ways for two guys to have fun. We basically just snuggled together in bed.
At a certain point, though, he just started saying weird or off the wall stuff. I do not know if he had taken some drugs or if he was experiencing a mental health episode. It was like he was hallucinating that he could see the night sky even though we were indoors in his room with the curtains pulled, since he kept talking about how pretty the moon was and things of that nature. And he would ask me the same questions over and over so I know for sure something wasn't quite right.