Instant kill glitch?

I invaded some gankers with a mage build using Velka's rapier just now.The phantoms both black crystalled out which was weird in itself.Upon seeing that I am a mage,the host casts one of the magic barrier spells.I backstab him for a whole 200 damage and he begins to turtle,it only took a sliver of his health bar.The next backstab deals a whopping 49 damage with the barrier still up and he loses all of his health and dies.

What the hell just happened!?

If you had read the whole post,you'd see I wrote that the first backstab took only a sliver of his health.A rough estimate is that he had 1200 left.He had full health when I first hit him.

Then don't make the assumption that I can't understand the health bar.This dude had just about full health and died from 49 damage,hence me posting that it glitched.I've never had that happen before and was wondering if anyone else has.