This is a phenomena I've noticed since I started healing 2.5years ago. Every now&then I would finish a healcast on a target, in range, in los, under fire. And the heal wouldnt land.
I've always written it off as "he just got hit for the same amount as the heal" since its occurred rather rarely. But recently I saw a thread on wow forums, and it turns out that alot of players have been experiencing this same phenomenon, and everyone has written it off as "he just got hit for the same amount as the heal" It would be funny if it wasnt so sad.

The reason I made this thread on ej is because in the wowforum thread, the cm asked for help replicating this thing, and unwilling to let this off into the hands of the wow community I'm bringing it up with the elite.
Now, this is something that all healingclasses have experienced I think, I've gotten confirmation from everyone except a shammy, but I would guess its no different for them.

As far as I know, the test parameters would have to involve healing someone in combat with a mob/player. I would have tested it myself, but extremely strapped for time these days.

The faster we document this, the faster blizz can fix it.

There's a multi-page thread about this in the WoW Bug Report forum. From reading through it, it sounds like there may be a legitimate problem, yet at the same time, it's hard to imagine such a bug existing in live without anyone able to document it and get that evidence to Blizzard.

I have been noticing the same thing, especially in arenas where such things stand out a lot more. I have yelled "WTF I healed you" more than once during an arena match. I always chalked it up to either a lag spike or the mentioned offsetting damage. Of course reproducing this will be hard but next time it happens I will check my combat log for evidence.

I've also had this happen. A lot, actually, and usually I don't see the heal land in my combat log despite the animation going off. I've actually been halfway through my next heal after a "failed" heal and my target dies.

This only happens to me when my target dies.

My latency is pretty consistently 60-80ms.

I'll try to keep it as simple as possible. It's really just based around the server. When your cast finishes, it sends a message to the server saying 'heal player x for y.' It's fairly common for the message to get lost or discarded, especially if theres high latency or high activity.

Yes, I can 100% certify this is not due to lag. The reason its not documented could be because you could need both the healer and the healee to log it. I'm pretty sure it shows up in atleast the healers combatlog.

Edit: Example from 1hr ago: Me and warrior duoing steamvaults, been doing it for half an hour when suddenly a heal doesnt land. (he was at 70% hp)
Cant be lag, we're both under 70ping. Cant be lots of action, we were fighting two mobs in an instance all alone. I honestly doubt its the instance server, GB isnt the highest pop server I've been on and atleast not when it comes to instancing.

And also, this is something I've been noticing for 2.5years now, across 3 different servers and (well its hard to say) not limited to any particular high-traffic timeframes or sumt.

Not dying, just the heal... vanishing, just like the title says. My girlfriend plays a priest and she swears this will happen from time to time, even in big, slow heal situations where the tank shouldn't really be taking exactly equivalent incoming damage. I think it's been written off as such, or raid frames not updating quite fast enough... but we consistently ping about ~50ms, too.

I'd be interested to see more about this but... I have no idea how this could really possibly be tested. I can't seem to find the thread on the forums talking about this, either, can someone dredge that up?

Delayed... yes. Lost? Thought TCP/IP guaranteed in-order delivery of all packets, so you shouldn't be able to lose the message without losing your whole net connection.
(I'm not a network programmer - I've done only one project where I had to do any networking protocols and it was a roll-our-own over wireless, so I could certainly be misunderstanding, but that's how I understand TCP/IP)

Our tank has complained of this several times in Karazhan. It's not just on deathblows, either. I remember one particular Curator attempt got borked because one healer's heals were landing fine, but another healer's consistently bugged out and the first guy couldn't keep up.

I realize the obvious response is "the 'bugged' healer was asleep and making excuses", but you could see his healing animation play. It's just at the end of the cast, the tank's health didn't go up.

I'm inclined to agree with everyone else that this is probably just bad packet loss, or Blizzard would have documented it by now. But it sure is frustrating when it happens.

Does WoW use TCP? I thought most games use UDP for enhanced performance. UDP reduces the response time and increases throughput but also does not make the guarantees that TCP does. However, if Blizzard is consistently losing traffic then they are doing something wrong or there is a bug in their server OSes networking stack. There are redundancy techniques to prevent this from happening even in bad conditions.

I'm fairly certain WoW would use a combination of various protocols. While TCP/IP is more reliable from a programming standpoint it isn't always the best option. UDP tends to be faster because its more of a "Send it and forget it" kind of architecture so it would be useful for things that are not crucial such as updating something simple like player position. TCP/IP would generally be used for something where you want a response from the server that says "hey I got that"

Anyway don't want to get off topic so back to the main point. My main priest sits 5 feet from me and has for 2.5 years, we've never noticed this phenomenom you claim.