Here's an example of that from Craigslist.
That page is encoded using ISO-8859-1 encoding, however, the web server is announcing that the page is in UTF-8 by sending down the following header:
Content-Type: text/html; charset=utf-8
This is a bug in Craigslist. It is a fair assumption that the Craigslist programmers do not know the absolute minimum that working programmers should know about Unicode.
Those curly apostrophes, in ISO-8859-1, are encoded using bytes which, in UTF-8, would not be valid. Thus they appear as
<?> in Firefox and squares in IE.
To fix the problem when you are viewing the page, go up to the View menu and choose Character Encoding > Western (ISO-8859-1) to tell the browser what encoding the page is really in.