Well, I'm a 4everalone virgin, so by default my worst sexual experience was masturbation.
For the first time ever (and last) I was fapping standing up in the shower. Partway through I felt a huge pain in the back of my head. It didn't feel at all like a headache. I was worried I was having an aneurysm or something. Dying with my dick in my hand in the shower in my parents' home is not the way I wanted to go out. Anyway, the pain in my head wasn't going away, so I gave up and got out of the shower. The next night, I went for a fap (laying down on the bed this time, per my usual) but when I started, the pain in my head came back. I stopped, the pain stopped. I started again, the pain started again. So I had to go a while without fapping. Eventually this stupid fapping pain went away completely.
But yeah, I guess I almost got a brain aneurysm masturbating. (in reality I have no fucking idea what the pain meant or was from, I just think of it as an almost aneurysm because I have no other answer).