WowAce

This bug occurs for most noticeably the Priest's Prayer of Healing and the majority of Resto Shaman healing spells. This bug does not however occur for Chaos Bolt, so it seems to be isolated to spells that were given new animations when Prayer of Healing and resto spells were. 

The cast bar will appear normally when the hotkey is pressed for the spell, but if it is clicked a second time during the cast, it will cancel the cast bar giving the indication that it is no longer being cast (if not noticing the animation is still continuing) while it actually is still being cast and this leads to confusion quickly if a spell is actually being cast or not. Any fix for this would be appreciated and if any more info is needed, let me know.

I'd like to add some additional info to help find a fix for this.

I added in some logging on my local version, and I notice that on some spells, WoW triggers a UNIT_SPELLCAST_FAILED event while attempting to cast certain spells repeatedly.  Like the user above, if I spam Prayer of Healing, I get a UNIT_SPELLCAST_START, and while spamming the button, I get multiple events for UNIT_SPELLCAST_FAILED.  The logic inside the event handler for that event, then causes the cast bar to then block out like the spell was stopped.

For temporary relief of the issue, I've commented out the handler for this event, but I don't know what ramifications that has.  The line in question is L#87 in modules/cast.lua.

Commenting out that line fixes the issue, but be wary as I don't know the issues it may cause.