I don't remember how old I was when this happened, but I think it was in my late teens (so not too young), but I was using my parents' computer for a little "me" time. I had started using Firefox recently so I installed that on my parents' machine and used it. I had a ton of tabs of porn open, and I just finished watching one of those preview quicktime videos where almost nothing happens (there was a guy touching the breasts of two topless girls), when all of a sudden Firefox disappeared. It was gone from the taskbar, and gone from the screen. It didn't show up in the applications section of the task manager, though I imagine it would have shown up in the processes if I had been smart enough to check there.

To be safe, I probably should have restarted the computer or something but for some reason I thought that the program crashed without telling me or some other sorcery that made it disappear forever.

Later, my mom calls me into the computer room asking why there is a picture (I don't think she noticed it was a video) of a guy touching two topless girls. Thankfully, I don't think she even knew what tabs were so she didn't see all the other (much more explicit) stuff I was looking at.

I could have given her the same explanation as you, and told her that Firefox must be a malicious program that finds porn and puts it on your screen, but I decided I loved Firefox too much to tarnish its reputation, and owned up to it. One of the most awkward and strange moments in my life.

I can think of another time in my early teens when I was going at it and I suddenly hear my brother and sister approaching. I scramble to get my pants all done up and the windows closed but one of those annoying pop-ups that only appears when you leave a site appears. I told them it just appeared out of nowhere, without me browsing anywhere.