Catching arcade cheaters.

I have a member that I suspect is using the Tamper Data plugin to cheat in my site's arcade. Naturally, before I call him on it I want to be able to do it myself to be 100% sure.

I can get it to work far enough to edit the POST comment, but once I submit it, I get a popup that says "Change 'Content-Length' header to %0?" with the options Ok/Cancel. Neither seems to actually submit the data, and I can't get my absurd score of 999,999,999 to show up.

Has anyone used this before, and if so what am I missing?

(Note: This is a community of webmasters, not petty children, so I'd appreciate relevant info rather than people accusing me of just figuring out how to cheat in a flash game, thanks.)