I'm not sure if it's my system, although I haven't done anything unusual with it, but I've started noticing incorrectly rendered characters popping up in web pages, text-files, [like this](http://ejohn.org/blog/html5-doctype/):

<http://www.kbssource.com/strange-characters.gif>

I have a hunch it's a related to the fairly recent trend to use unicode for everything, which is a good thing I think, combined with fonts that don't support all possible characters.

So, does anyone know what's causing these blips (am I right?), and how do I stop this showing up in my own content?