I have earned a few hats, and I went to set one. Worked fine the first time, but I got the positioning way off, so I went back in to change it, and got this:
An error occurred while changing the hat - please try again.
I was able to hit the remove all hats button to clear the bad positioning, but it won't let me set a hat any more. I've even switched sites from Arqade to here, and tried again, and it's still erroring out.
This is in Windows 7, Internet Explorer 10, fully updated, with no plugins/addons.