I started a version of this post about a year and a half ago, when users of a CMS tool I'd written noticed strange characters in their documents. Quotation marks were showing up as foreign letters. The legal section symbol (§) was showing up as the degree sign (°).
You've probably seen this problem yourself in email or when copying and pasting web content. The text looked fine to the person who gave it to you, but to you, it looks like garbage. Why? As one of the clients using my application asked, "It's just plain text! What's the problem?"