I just lost a full gear set because my bullet did no damage. What happened?

Here is the combatlog If you need some help understanding what the combat log means, he headshot me, I hit him with my crossbow lowering his HP to 79.3 and shot him with the Python lowering his HP to 30.9, and then I shot him again with the Python, but this time it did no damage so his HP stayed at 30.9. Since the second shot did no damage he was able to just waterpipe me. Is this a bug? Lag? Has this happened to you guys before?