This happens to me sometimes when I click a lot of things quickly while stuff is going on. My suggestion is to wait until each action is completed until you start issuing orders for another.

i finally hit this bug for me it triggered when a unit(in this case hero) attacked and died to the counterattack. it also happened to kill the last enemy unit at the same time. autoresolve is unclickable. end turn doesnt work and unit animations are frozen.

same to me. FIX it! 

Happened s to me a LOT.  Mostly after one of my melee units swings.  I can reduce the chance of it happening by waiting longer between clicks (like Heavenfall suggests above).  Doesn't seem to matter if I click too fast when my archers are shooting, though.  So I think it is timing-related in some part of the code path that is separate for melee and bow attacks.

And I'd vote for fixing too, but I think it needed to be done before most of the team moved over to FE.  If Brad is the only one still working on WOM, I think we are SOL because this is probably outside his area of competence -- they would have to pull a coder off of FE for a slow and frustrating debugging ordeal.