Fake virus warnings. They are designed to look real by making false claims of being associated with Microsoft, have a blaring "siren" sound effect, having a voice come on and tell you that your computer is infected, making it almost impossible for you to navigate away from the page (I always have to shut the computer down), etc. Why they are fake: 1.Your anti-virus software won't detect the so-called "virus". 2.Surprise, surprise; your computer still works just fine! 3.They will usually direct you to a fake company where a fake technician will perform a fake scan on your computer that will always show that you have viruses, even if there aren't any. Then, the "technician" will claim that he/she can "fix" your computer if you pay him/her. But this is really just a way for them to steal your money and identity by getting access to things like your phone and credit card numbers. I came very close to falling for this scam once, but once I told the fraud that I didn't have the money, he gave up, and I was allowed to get out of there, thank god.
Fake virus warnings also pop up on cell phones a lot, too. Common messages you might get include: "Your phone has been infected with 4 deadly viruses!" (Then why the hell does everything on my phone still work so well after all of this time?) Or: "Your phone has picked up a virus from visiting adult sites!" (Adult sites!? I was on a site that lets you download anime music, not porn!)
Anyway, they seem to be popping up a lot nowadays and are really annoying. Sometimes, they will permanently block videos online, and that is really annoying when you are just trying to watch some anime, like me. If you ever encounter something like what I have already described, the important thing is to NOT panic, because chances are that it is likely fake. Also, I recommend that you try shutting down your computer immediately to escape them, because it can make exiting a page extremely difficult, sometimes even impossible if you get too far in. Also, when you come back on, do not choose an option that will let you go back to where you were or recover your progress or whatever because then you will be right back where you left off with the fake virus warning. I have heard that it is easier to escape if you are using Google Chrome as your browser, but I don't know if this is completely true.
Maybe a lot of people on here already know about this stuff, but I thought that I would post it anyway just in case. And also because fake virus warnings are annoying and I hate them!