I'm going to throw this personal anecdote from a few years ago out there for whatever it's worth. I was on the Full Metal Alchemist wiki for whatever reason and I had scrolled down to the bottom of the page where the advertisements were arranged. I assume what ever I saw was part of an advertisement but I'll never really know because the next thing I knew I was scrolling back to the top of the page and my head hurt abominably. I'd lost what I guesstimate to be three second of my memory or at least my visual memory. I navigated away from the page and spent a few minutes away from the computer. I'd experienced that sort of pain in my head before; it was caused by all of the muscles in my head and neck tensing at once. I went back to the website pretty quickly (my curiosity will always win out against existential dread in the short term) but it never happened again.

I can only speculate on what I saw. I know that there were a few ads for charities to help kids with birth defects like cleft lip and worse and some of those images bothered me at the time but that was the most disconcerting thing I could think of at the time and hardly seems the sort of thing to have a dissociative moment over.