Company somehow screws up the dropdown for their credit card expiration date to include the LETTER O INSTEAD OF THE NUMERAL ZERO for September.
You can guess what month one of my cards expires in--the server-side validation failed and I had to use another card (though I do realize I could have edited the selection with Firebug or the Web developer toolbar).
Making it so 1/12th of your customers can't make a purchase from your website: priceless.
(It's also awesome that you can still enter cards from 2003. At least we know the site has been in this state for the last four years...)