Something I realized recently is this: If you switch to another item in your hotbar, the first hit you get will not register. I tested this by having my friend stand perfectly still in a 1.9 duel. Switch to food and then back to sword, swing at him at point blank range and the hit doesn't register. (this is not lag, I waited a few seconds it never did register) The 2nd hit, however, does register. I'm assuming this has something to do with anti-cheat. If it's a bug, please look into fixing it, you have to switch between items a lot in PVP, so having your hits straight out not register is annoying. If it's intentional, I got to tell you that this is a stupid feature and is more like "anti-skill", not "anti-cheat".