I've never heard of this problem. What operating system are you using, and what version of Java? You might try reinstalling Java and the game. Perhaps some of the game files are corrupted. Also, do you run through Steam, or use the stand-alone client from www.spiralknights.com, or what?

While we wait for someone more knowledgeable to post an answer, you might want to ask Three Rings for help. There are two ways. First, in the game click on the ? button in the lower-left corner of the screen. Second, move this thread to Technical Issues (by editing the original post and choosing that forum from the drop-down menu). Good luck.