Beware of narrow fonts
One weird problem that I have observed a few times is that a website has been shown with a narrow font on one computer. While you can say "who cares" as long as it is only one computer, you only test on a few machines anyway so one might be a significant percentage and indicate that you should do something about it.
After some digging I have found that the problem in these cases has been that the computer in question does not have the regular version of your preferred font-family, but it has a narrow version of it. While I don't know if this is according to standards, the computer/web browser tries its best by applying the closest installed font, hence the narrow version. This has occured with the fonts Liberation Sans and Open Sans. The first occured on a Windows machine, the latter on a Mac (Open Sans used with Google web fonts, browser still applied its' locally installed Open Sans Narrow).
While I don't have a solution for this, a known cause makes it easier to evaluate whether or not a workaround for this font-family problem is needed.