What's the deal with being insta-killed by a zombie when it lands an attack from behind, even though I have full clothing on (some armor included), and the military backpack on my back, but can still be one-shotted? I mean, I think it's fair to say that with such a large backpack on, reaching the neck area should be incredibly difficult.

Do you have your infection settings set to kill instantly? Either that or neck wounds will get you quickly.
Back attacks are much more likely to be worse, get through armor/clothes and be bites instead of scratches.
Agreed about the backpack though, should make it a lot harder to get you

were you sitting down or was in the process of getting up from sitting?

I've been one-shotted once from a zombie biting me from behind while I was sitting on the ground. I thought it was simply because they got me when I was sitting. other than that, I've been bit from the back when roaming around and survived it.

First of all, always check your mods. It could be one of them breaking the game's mechanics.
If you are playing purely on vanilla, check your survivor's condition and your game setting as mentioned above.

I recall the devs mentioning that there's actually a bug that can cause an instant-death. However, no one has been able to accurately replicate the issue and consistently, so they are having difficulties finding the root cause.

it happened only once to me, and a few days ago. Since I have the Cheat menu mod I was able to prevent my death, and weirdly enough none of my armor was damaged (some clothing always tear up when you get attacked regardless if it protect you or not, normally).

Died while standing still (I had an urgent need to sneeze irl), hear the death sound immediately after the bite sound. was really weird.

Did you set your zombie infection to instant?

answer is check your rear. The reason you died isn't game mechanics, it's you being overconfident and not checking behind you. 99% of the time in this game the real reason people die is because of their own mistakes. I am in no way immune from these mistakes.

Most likely you got a Deep Laceration in the neck.
Deep laceration in the neck you hp drops by 25% per second

Has happened twice now from a runner zombie. They land an attack and boom insta-killed: once from behind and once just now from the side, and I was standing on both occasions. The infection is set to 2-3 days.

Could just be a mod.

One of the default settings in the game files is that you are naturally susceptible to critical damage from the rear. you can see this if you go into the sandbox mode options. The option is called Rear Attack Vulnerability. There is also a small percentage chance (less than 5%) that you can suffer an instant kill from a neck bite from the rear.

that's never happened to me before and I've played this game for many years you found a feature not a bug

Can absolutely happen. Fatal bite to the neck is known to happen. It's rare, but I've had it happen.

Originally posted by The Toblin:

Can absolutely happen. Fatal bite to the neck is known to happen. It's rare, but I've had it happen.

Yeah it is extremely rare but it can happen. Only way to protect yourself from it is wearing a motorcycle helmet. It will not let you be damaged, but instead falls on the ground. (But only works if it goes for neck or head, other parts of the body are fair game!)
Is why most folks prefer keen hearing.

wear a scarf, put padding on your jacket. do a quick 360 ever 5 seconds lol

Originally posted by red66:

wear a scarf, put padding on your jacket. do a quick 360 ever 5 seconds lol

Scarf does not protect it. It has 10% or 20% chance to mitigate the incoming damage. So a bite turns to laceration, but it can still kill you pretty fast if it is a neck laceration. My least severe laceration gave me bandage change +5 seconds to live. The worst less than 4 seconds.
Only the motorbike helmet (and i think there is another similar item) that gives protection against this. NOTHING ELSE.

Originally posted by Nordil(Hun):

Originally posted by red66:

wear a scarf, put padding on your jacket. do a quick 360 ever 5 seconds lol

Scarf does not protect it. It has 10% or 20% chance to mitigate the incoming damage. So a bite turns to laceration, but it can still kill you pretty fast if it is a neck laceration. My least severe laceration gave me bandage change +5 seconds to live. The worst less than 4 seconds.
Only the motorbike helmet (and i think there is another similar item) that gives protection against this. NOTHING ELSE.

Every little bit helps, better to wear one than not.