Causes of failed hanging?

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What can cause someone to fail to ctb when hanging themselves (full-suspension)?
I know that being found early enough can be a reason. And if the ligature wouldn't be able to hold the weight of the person they would most probably fail to ctb. But are there any other things that can make someone survive?

 

[lemon17]

A lot of potential reasons: weak and broken rope, unstable hook in the ceiling, non-constricting noose, too long rope (then you will hit the floor with your feet).

 

[CrappyMJ]

Make sure the rope is strong enough. I tried partial a week ago and after I was unconscious, the rope snapped. And try the knot with your hand, put your hand in it and push as hard as you can.

 

[JadedGray]

Survival instinct kicking in.