Something similar to the above seems to be happening with me. I took a screencap of it so you can see exactly what is happening.
1. Operating System: Windows XP SP3
2. Browser: Firefox
3. Tried Browsers: Chrome and Explorer - Same screen came up.
4. Problem Started: The problem started some time after 11:00 EST yesterday night.
5. Username: Darth_Darkrai
6. Problem:As you can see from the above image, this error message pops up whenever I try to access showdown, or try to reload the showdown page I still have open in a tab. As far as I know, nothing changed on my end since yesterday afternoon, when Showdown was working perfectly. So, either something's wrong somewhere along the connection line, or I lost so badly yesterday that the server just refuses to let me back on. Frankly, either is possible.
EDIT: Additional Information: I just discovered that I can get onto the server with my downloaded copy, but still can't get on from my browser. Since the downloaded copy slows my computer the f*ck down, it's still rather inconvenient. I'm not sure why I can get on from the program, but not from the link on Firefox.