I'm sure everyone here has suddenly been one shotted from near max HP by something that does less than half your HP. That's a HP desync and they can be annoying and ruin your fight, so its important to be able to identify them.

Firstly, you need to know what causes HP desyncs. Desyncs happen when the server fails to update you on a damage source that you've taken, and as a result the HP displayed on your screen is higher than it should be. Eg, your HP can be displayed as 80% but in reality its at 20%. Now, this does not mean that your Max HP is permanently lowered. If you are desynced by 50%, that just means that your HP bar on your screen won't start dropping from 100% until your actual HP falls below 50%, and you die once it reaches 50% on your screen. An easy way to visualize this is just to add 50% to your current displayed HP, but if the result is more than 100% you just can't see it on your screen.

There are 3 ways to identify a HP desync:

1) You are at max HP and take damage, but your HP bar doesn't budge. This is the easiest way to tell that you are desynced. The more damage you take before your HP bar drops, the more severe the desync is.

2) Your HP is not low on your screen, but your screen turns red and sound distorts as if you were near death. This will tell you that your actual HP is very low and you should quickly heal back up to full on your screen and KEEP healing as your actual HP might still be low.

3) If your group members notice that you are not healing/playing defensive until you are dangerously low on their screen, that likely means you're desynced. Similarly, pay attention to your groupmates; if you see them not healing until their HP is lower than it should be, they might be HP desynced.

Common stuff that cause HP desyncs:

1) Ganks. Since the opponent lands a large number of hits simultaneously from stealth, this can cause desyncs especially if it is laggy that time. After you survive a gank, pay attention to your max HP bar as you might be desynced.

2) Shock damage. For some reason, shock damage seem to cause desyncs more often than any damage type. My theory is that the extra damage you take from Minor Vulnerability proc'd from shock abilities may fail to register on your screen. If you're fighting someone with lots of shock damage, such as an overload magsorc or someone using shock staff with shock clench, pay more attention to your HP bar.

3) Overload. Like all shock damage, overload tends to desync your HP, but for some reason it is much more common to get desync'd by overload than other shock damage.

4) Overload+Elemental weapon. This one is GUARANTEED to desync your HP. Every magsorc I've fought using this combo has managed to desync my HP, usually quite severely, and likewise when I use overload+ele weapon on my magsorc I've managed to desync pretty much everyone I fought. It has gotten so severe my dueling guild banned magsorcs from using this combo in tournaments. If you're fighting a magsorc using overload+ele weapon, pay attention to how badly you're desynced as its not a matter of whether you'll be desynced or not anymore. The longer you fight them for, the more severe the desync gets. It can even be as bad as 100% where you just die instantly at max HP on your screen. My theory is that because of how clunky overload is, the dmg from Elemental Weapon tends to not register.

How to remove HP desyncs:

Sadly the only way to remove a HP desync is to die. Should you notice a HP desync during a fight and survive that fight, I recommend killing your character afterwards to remove the desync for the next fights.

Reverse HP desyncs??

Theoretically, a scenario where actual HP is higher than your displayed HP is possible, likely caused when a heal doesn't register on your screen. This might explain some situations where your HP or opponent's HP drops to 0 and they still don't die.