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It's happened a couple times, I have no idea how to reproduce it, but occasionally the Masteries page in the Summoner Profile will bug out, and won't work/respond. If I remove points I get points out but can't replace them, and the points aren't removed, I just get back the points I used again. Can't use those anyway though, so it's not like an abuse bug. The problem resolves itself if you relog the client but... well a bug is a bug.