Button mashing a spell even though it doesn't cast because of (not ready yet, not even rage, not enough energy, etc) will trigger a GCD. Drastically effecting chaining spells together (mashing mortal strike so you trigger it as the cooldown occurs, arcane explosion spamming will actually have delays betwene explosions if you press the keys to fast, prayer of healing.

http://www.wowinsider.com/2007/11/05/dont-mash-in-2-3/

I wonder if this means the world is going to move to hardware macroing tools and there will be a backlash crackdown on people that server up multiple perfectly timed spells.