I finished the quest nobles in exile and got a bunch of indicators pop up on the left. One such was the new hero that joined my army and my barracks popped up saying it was full. Rather than immediately choose a unit to disband (which I should have done) I started clicking the other indicators. One such was a construction complete message, which took me to and selected the place where it happened. I suppose because my army was no longer selected then I couldn't select a hero to disband. Rather, I could select it, disband it, and nothing would happent. Unit inventory still said 19/18 and the unit would not get deleted.
So I figured I'd just select my hero again and all would be good. Both my heroes were greyed out so I couldn't select them.
Basically, It stopped progress. I couldn't select a hero because of the unit list overlay, and I couldn't close the overlay because I didn't have a hero selected (at least, I think that is what the problem was but I'm just guessing). Had to alt tab and to a hard close.
No big deal, I'm sure it's pretty rare. Just wanted to report it in.