Found this out last night, very annoying.  When playing Sedna it's possible to cast heal when the recipient of the heal is currently unable to be healed.  I wasted a heal numerous times because I cast it right before my ally went into Mist form, and other times when I tried to heal right before he died, was too late, but the heal went off anyway. 

The game should check before the cast goes off whether the recipient can still be healed; it shouldn't be possible to get a heal off and have it not affect the recipient, and it DEFINITELY should never be possible to waste a heal on somebody who's already dead.  I'm all for forcing players to be aware of timing, but this business is a little bit much (and I can only assume it's not intended).