A Fire mage (or any 85 mage with enough talents in the fire tree) can choose the Impact talent. This talent has a 10% chance to remove the cooldown remaining on Fire Blast when using damaging spells. If for some reason you get Counterspelled (or some other interrupt Silence) on your fire spells and this talent procs, Fire Blast is directly useable again even though it shouldn't.
At first I thought this is working as intended but then again, an interrupt silence is supposed to make sure a school of magic is unuseablle for x amount of seconds. So it's a minor bug that's very situational. Though for Fire PVP mages I guess it's usefull.