I was browsing the I-net recently when my malware popped up a 'blocked page' notice. No surprise there- we hang out in strange neighborhoods, we find some bad stuff. Almost immediately after, a full-page banner popped up. Red headlines, blue-on-pale grey text. It claimed to be from the FBI... "we know you've downloaded illegal pornography, we know you have ch1ld pron on your hard drive" etc,etc. I actually felt a moment of panic when I read that they were going to 'lock down' my computer and other horrible things.
But then they offered me an out- if I bought $500 worth of prepaid debit cards and mailed them to so-and-so address, they would consider my fine paid and... You know the rest.
By this point, I knew it was BS (of course). My only real problem was getting the banner off my screen.