1. Users who don't read the instructions on the screen. Doesn't matter how big you make the notice, or how non-technical and succinct you make it, they still won't read it. They prefer to click, get stuck, and then phone for help.

I did that.
Was getting some HTML software off the web. Downloaded, installed. Didn't work. Updated. Didn't work. Uninstalled. Uninstalling triggered an auto-form asking if I wanted to tell the company why I'd gotten rid of the software.
So, I decided to fill it out. Said I couldn't get it work on my system.
Half an hour later, I got an email from the developers explaining that the latest Windows security update interfered with the software's function, and that there was a patch available.
Downloaded and installed the patch, and *poof*, it worked!
I emailed them back, thanking them for their help and suggesting that they put up a notice on the download page that you need the patch.
Quick reply: There is one.
Went to check. Didn't see it. Decided to take a more careful look.
And there, at the top of the page, in giant red "this is hugely important; read it now or you're an idiot" letters, was a notice about the problem and the patch.
The reason I didn't see it? I'm so used to that sort of thing being used to sell virtual snake oil that I automatically block it out. The content was recognized and disregarded before it ever made it to the conscious level. I had to stop, realize there was a part of the page I hadn't actually looked at, scroll back up, and consciously force myself to override the mental filters before I could actually read the text.
Oops.
Paul