Zero Stress is one of the methods to suffer an injury. There are other methods of suffering injuries.
Stun phaser attacks or nerve pinches inflict non-lethal injuries.
Both player characters (including supporting characters) and non player characters are knocked out with the first injury they suffer, regardless of type, p. 176, 177 of the Core Rule Book. If it is a non-lethal injury, a second non-lethal attack will transform the non-lethal injury to a lethal injury instead of adding a second injury.
If the character already suffers from a lethal injury and suffers a second lethal injury, they die immediately. If the character suffers from a lethal injury and suffers a non-lethal injury, they have a lethal and a non-lethal injury. It will take another injury (regardless of type) to kill them.
Injured characters will, at the end of a scene (p. 177),
- recover if the injury was non-lethal,
die if the injury was lethal and they did not receive first aid.
Note that there are ways to either avoid injuries (so they never happened) or ignore injuries (the character is injured but not knocked out), depending on character type (player character, supporting character or non player character) and narrative currency. See p. 176 f. for more.
Mean Right Hook and Nerve Pinch do not stack, as the former affects only unarmed strikes (p. 137) and the latter counts as a melee weapon (p. 111).
Hope that helps.