After seeing a post made by Vargas about this, I feel it is time to come forward with my findings in the hope that this can be resolved.
I was on the duel server teaching my girl to parry. I would hold an attack direction and wait for her to parry correctly before releasing. We were sitting next to each other so I could look at both screens.
A strange thing happened. I had not released my attack, as on my screen the parry was in the wrong direction. My gf asked what I was doing, I said that she was blocking the wrong way. She assured me that she was blocking correctly, so I looked at her screen.
On her screen she was parrying left (the correct direction, as I had a right swing chambered). On my screen, however, she appeared to be downblocking. When I released the attack, it hit her as it should have given what I saw. However, on her screen, the attack went straight through her parry.
I was surprised at this and put it down to lag and we kept going.
In the next 10 minutes, it happened maybe 5 more times. I cannot recall the specifics (ie which attack/block directions they were. It may have been the same ones but I'm not sure.
However, each time it happened I got her to wait and even run around a bit before I struck. This was to test if it was due to latency, and I do not believe it was.
After it had happened a few times, I got the rest of the players to come and watch it happen. Maybe someone else can confirm, as I did not have the time to do more exhaustive testing.
Moral of the story: there is a bug that will cause an enemy to swing through the correct parry. When I realised this, it made a lot of sense. All those "I swear I parried right....." moments have been vindicated.
Shpoing
I neither claim to know how it works or how to fix it, but it does exist. Of that much I am certain.