I turn off my computer when I get one of those (just because I'm scared to click on anything in those situations)... I didn't know closing the browser would be sufficient. And while I don't click on the little upper-right-hand "x" when I get a pop-up because I just assumed that would be the same as clicking "update" for a malware-type pop-up, I didn't know the back button would be a problem too (and luckily for me, I never tried it).

Thanks so much for the warning!