I returned for Classic and I’ve got an situation that occurs every so often when I’m healing.
Sometimes I will cast a heal spell, the cast bar timer and animation will go and after 3 seconds the cast complete animation and sound plays (and I think my mana goes down), but my heal target’s HP does not go up. This only happens once per instance or so, but when it does it is obviously a serious issue as the expected big heal never landed and now I’m scrambling. I’ve had this both in groups and using raid frames. Why does this happen? I’ve got a few possible ideas, but none sound quite right.

1. It’s some connection glitch that causes my client to think I’m casting a spell, thus playing the animation, but I’m really not. It explains the lack of heal effect, though seems like the kind of thing that should be gone by now unless this is a Classic “feature”.

2. Something related to Classic Batching. Several people told me they have seen the same heal cast failures, they attribute it to this. I don’t get how (maybe I misunderstand batching?). This heal failure happening on fights I’ve done a ton and know there is no way for the mobs to do a spike that large within 200ms, let alone the coincidence in the spike damage exactly canceling the heal amount to no visible effect. This also isn’t talking a tank who’s moving around and maybe getting out of LoS or range at the wrong moment, which if anything should be more forgiving if batched.
   Ex: The last time this happened I was healing a tank in Stockades who was standing still in the middle of the hall. Those low level Defias have no way to spike +400 damage in a fraction of a second to negate a 400hp heal.

3. Some addon bug causing my spell to not cast or land on another target (say myself) despite the tank being targeted and my finger being nowhere near Alt to self target.

Can anyone explain this and how to avoid it? Thanks