I am not sure if this is strictly Mac client issue or not, but when I level up and go to my rune page to put a new one in, it usually stays locked for a match or two after I actually level.
I.E.: Level 11, says Level 11 Mark Slot is Locked.
Machine Type: MacBook Pro
CPU: Intel Core2Duo 2.4ghz
Graphics Card: Nvidia 320m
Settings Levels: Everything high, shadows etc.
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