Hmmm... Expressed myself a bit bad.
It was a Bug when it occured, you was able to shot
the enemy infinite without killing it.
I suppose it was a Bug in the way, the Hitpoints were taken.
Probably they were compared to zero rather >= zero, so that
the enemy health became negative values in rare occasions.
Or the trigger for the get-killed sequence was programmed to allow some situations to not be activated when enemyhealth was >= zero... Dont know the code, so dont really matter...
Anyway, in rare situations an (always different) enemy became immortal.