One of my cities, a large one enchanted with Wall of Fire and Blood Sigil, just got attacked by a Fell Dragon (which has a Fear effect that triggers on battle start).  All the effects triggered, and while my units got Feared and Berserked and their units got burned by the wall, the Wither effect didn't resolve.  Its card still showed up on the left hand side of the screen; the mouse cursor was the "wand" for casting spells, but clicking gave me a dialog saying that the action was invalid.  I gave it a minute or two, but the game seemed totally stuck.