Hi there, wandering in the bug reports section, I found your post.
This happens to me sometimes. So frustrating. I have tried to hunt this with fraps to get video/pciture as evidence. As you say, it is not always clear in the heat of a fight wether I miss-clicked, blocked the wrong way, etc... So far I could not get an obvious picture showing an attack going
through my block
All the videos I have made show my character not starting the block even though I was sure of clicking RMB correctly (especially when the attack is obvious and very predictable). And this concurs with what my opponent saw: for him, I just did not block.
Also, I have to say that, on my side, I have never seen one of my attack going through the block of an opponent.
My theory is that there are packets of information that are lost through the connection. So you might block correctly but the server does not record the block in time or lose the information. This is just my theory, I can't prove anything.
Fortunately this happens not that often to discourage me from playing