I was playing last night and noticed that from time to time I couldn't throw a heal on anyone (including myself) until I moved from my spot and then tried the spell again.  I wasn't able to replicate it but it happened a few times and was the same thing each time.  I'm not sure if it's related to how long I stand still in one spot but I was able to heal most of the time correctly... just ran into a few bugged occurances.

It's insanely annoying when it happens though as it always seemed to happen as I was trying to heal against a damage spike =/