Recently if you tried including a font from another domain, for instance a CDN things were fine unless you were using FireFox http://logicalfriday.com/2012/03/21/cross-domain-font-woes-in-firefox/

Recently this behavior has started showing up in Chrome as well which means that it will requires access control headers to be set when requesting fonts across different domains.

So if you’ve suddenly noticed your site not looking right this might be something to check.  Most font repositories like Google Fonts will be fine, but many smaller ones or if you’re hosting things on your own servers/CDN’s this is something to be cognizant of.