It is possible to cut down some of the larger oak (and possibly other-unsure) trees and lie down in them to be invisible and blocked from all incoming bullets. The exploiting player inside the tree trunk can see out and fire his weapon, but cannot be seen or hit.

Example:
https://www.youtube.com/watch?v=jA9Yzu7stfU&start=60

Well known streamer (who I won't name unless this kind of abuse continues) is seen in this clip exploiting this bug. He lights a fire to lure them, then lies prone and invisible in the tree trunk. He kills the first guy in the duo and then his friend is unable to return fire even though he knows where he is.

This is blatant abuse.