Client and server desync I reckon. At client it appears as if you're holding down block but at server it's registered as a right block for example, thus making the enemies attack (who sees a right block because the server told him so), go through your "down" block.
Same can happen when you're actually holding the wrong block, but on the server it's the right one (because that was your previous block) and you actually block for example a left swing with your "down" block. As soon as that happens your block changes to the correct one without you even releasing right mouse button, which is really funny.
Same can happen with chambering weapons and releasing left mouse button, if at the moment of left mouse release there's some lag/packetloss/whatever and then when you're not holding down the mouse button you still have your sword chambered. I specially noticed this one when cmp implemented the delay to 2h right swing chamber, my sword got "stuck" in chamber quite often.
I'm sure it has happened to quite a few others as well, I get this now and then.