Basically, I was using a Divine Avenger on some Poison Zombies in Depth 19. I hit one while it was coming out of the ground, and thus it didn't take any damage. Then I landed the second hit of the combo. Something strange happened. I noticed that hitting it again with the first hit didn't kill it, but comboing successfully with both hits landing would kill a zombie on the level instantly. I also saw that the chunk of HP taken out of the gauge was a lot less if I only landed the second hit.
Is there some hidden mechanic I'm not aware of?
I will look into this. Could you tell us what level of damage bonus you were at?
Were you alone for the whole thing?
Were you poisoned when performing the combo?