Vayne Auto-Attacks Deal No Damage?

I was playing an ARAM a few minutes ago and on two or three separate occasions our Vayne's Auto Attacks did no damage to a target who would have died otherwise. She was alive when the attack connected and the target was clearly in vision with less than 100 HP remaining before the attack. I'm not sure what caused this, but it was pretty interesting to see (infuriating to the Vayne though). At one point the attack also activated her passive and still did no damage. So, that was a thing as well. I just thought I'd mention this somewhere that it could be seen because it would be a pretty major issue if this happened in a big match or even just in ranked queues.