Just looking on reddit and found some threads and some "interesting" youtube videos.

Apparently since the masteries got moved from client to server side there was an exploit where you could put more than the max number of points in a mastery, for example there is a mastery that reduces summoner spell cooldown by 15% however the exploit allowed you to put 7 points in this so 105% cooldown or 0secs.

This lead to the following http://www.youtube.com/watch?v=RkqVQ1aYKVE ( pretty rofl to watch but imagine in your ranked game).

http://www.reddit.com/r/leagueoflege..._on_summoners/

Since this has been on reddit Riot have hot fixed the problem, so hopefully we won't see this on the EU servers.