I've recently noticed that there is a seriously annoying and combat breaking bug where, for whatever reason, my character is unable to light attack, heavy attack, or cast any abilities in PvP, and he is permanently slowed. It is only fixed by pressing block, or on some rare occasions by hitting escape. It seems to be triggered by ultimate abilities, but it could be triggered by other things as well.
The reason it is combat breaking is that it occurs very frequently, and often occurs when I am chasing a player down to kill them, or after I have been cc'd (typically an ultimate is involved). Typically this causes a failure to defeat the opponent or a failure to escape the pounding that follows the CC.