Upon logging in to my account, I discovered that I had a total of -3809 crowns on my character. Prior, I had around 500 to 1000 left.
I play Spiral Knights through Steam, if it makes a difference.
Here's a screenshot of how it looked:
http://dl.dropbox.com/u/3411110/fml.jpg
You can see the amount of crowns in the bottom-right of the screen above the CE counter.
I have noticed multiple reports on this throughout the forums, yet no answer has been found to its cause.
I hope that this problem can be solved.
Thanks.