Someone thought it clever to register with the site last night using the username "The-1337-Hacker" and then proceed to hack a load of the games in the Arcade to give themselves extremely high hiscores.

The hacked high scores have been removed from the arcade and all existing score rankings restored. If you received a notification that any of your own scores were beaten by The-1337-Hacker please disregard them as your existing scores stand.

This member has been banned from the site and their activity has been directly reported to their ISP as a breach of their AUP.
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So how did he proceed?
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I can't say here, publicly, but I do know.
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Probably he's not completely unknown.
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