I can't say its the most inappropriate thing but the first thing that came to mind when I saw this thread was an experience I had when I was young and on the interwebs (Shockingly). I was probably 2nd or 3rd grade and before I knew about Newgrounds I used to go to a website called Flashplayer.com which was basically the website version of people who reupload other people's Youtube videos. I think actual authors also uploaded content but a lot of it was just stolen .swf files from Newgrounds reposted on this website. The site doesn't even exist anymore. But anyway I would go there to play some fun flash games: Bowman, a worms clone remember the name of, and Madness Interactive, which was way too violent for me at that point in my life.
Anyway, me and my brother who is 3 years older than me would often go to this site and I remember one time he had something he wanted to show me so I sat and he rapidly typed in flashplayer.com into the addressbar of like Internet explorer 5 or something. Except he forgot the -ER on flashplayer, and at the time flashplay.com was one of those really annoying pop-up adware porn sites because he hit enter, got redirected to some porn site, and I specifically remember a popup that had the stupid "Are you sure you want to close this page?" thing which was basically just a 30 second loop of a woman performing oral sex on some fellow." Again, I was like 9 years old and my brother was probably 12, neither of us wanted to watch such a thing in each other's company, and I didn't want to watch it at all at age 9. So I sat there as he frantically tried to get the stupid pop-up to close and the brain makes things seem longer but in my head I think I watched this clip for about a minute. In retrospect it couldn't have been more than a few seconds but I still remember it lasting much longer.
It didn't really do to me at the time, but I have noticed that over the years, that fear of being in a situation with family or in public where my computer starts spamming porn pop-ups and I can't close them in time recurs often. I even had one earlier this week, my cheap laptop had become virus filled and I took it to a library on campus and when I opened it up it just spammed me with porn popups and other garbage, I found that a bunch of stupid programs had installed on my computer that changed all my icons and homepages.
It is a very real nightmare I have a lot and I think that this specific incident is the culprit.
Only other mention is again around the same time period, I definitively know it was second grade because of my age and the when GTA3 was available on PS2. My brother's friend had GTA3 and he got to play it at their house. I wasn't invited and was very pouty about how my brother got to break the rules. Somehow we convinced our parents to let us rent GTA3 from the local Blockbusters and then we went home and took turns seeing who could survive longest, given that we both would immediately type in the weapons cheat and start shooting stuff, then flee in a car from the cops and then die. After that we handed off the controller and tried to top the previous attempt in length or Michael Bay-ness. Respawning at the stupid hospital so many times, and the horribly 'realistic' animation for getting a headshot with a sniper rifle gave me an interesting nightmare that ultimately made me stop playing violent games for several years. For people who don't remember/know, in GTA3, the scoped M4 or the sniper rifle allowed freeaim which meant you could shoot people in the head. When you did that, their head would disappear, they would spray a fountain of blood and stagger around like zombies for a few seconds, and then collapse into their own little pool of blood they created. I started getting nightmares where I would see the panning shot the game does when you respawn at the hospital, but when I got control of the character "Claude", all the people on the sidewalks would already have their heads shot off, and were in that horribly rigid, staggering pain state of being blood volcanos. They would never fall down but just kept on staggering in inhuman agony.
I had that dream one night and that was it, I remember crying in the bathroom trying to explain to my mom how I wasn't grown up enough for the game, and explaining how scary the headless people were, it would have made a great youtube video. My parents returned the game the next day and I had a very pissed off older brother for a while because he never even made it off the first island in the game.
Wow that was a long post.