The error that appears in a little red box:
An error occurred while changing the hat
My winter hats broke after I added an account that had an old question linked to it. During the merge process it recalculated most (but not all) of my badges and most (but not all) of my winter hats. Since then I have been unable to wear a hat. I just got the Odin hat and I'd love to wear it. It shows a hat for me on MSE but on StackOverflow.com it does not... and in either case, I am unable to wear it (or any other hat) and will receive an error.
Here is the original thread where I figure out how to add another account that subsequently broke winter hats for me: