Actually, event handlers are pretty much a "magic wand" that can create invincible units. Like Knut Erik said:
this addEventHandler ["Hit","(_this select 0) SetDammage 0"]
put that in the init field of a unit, and it becomes invincible. You can even make yourself (the player) un-killable with this. Try to see how far you can blow yourself away with pipebombs. It's rather fun to be flung 100 meters through the air by a huge explosion, then just walk away ;D
I guess there are a couple limits, though. First, a unit has to suffer at least 5% damage in a single hit for the eventhandler to go off. So an MG bullet hitting a chopper isn't going to set off the eventhandler (but a stinger will). Also, if you increase the time acceleration to x2 or x4, sometimes the computer won't register the eventhandler on time, and the unit will die.