Not sure if "bug" but I have found myself in a situation in which I had a Champion trying to cast Blizzard and when the moment to cast it arrived... not being able to cast it because there was only one enemy left and there was no way for me to target only him.

The spell requires to target at least one enemy unit and must not target any friendly unit. My case didn't allow me to actually use the spell as the enemy was surrounded by three sides.

I tried pressing Space, Esc... tried to target empty land... I found myself unable to cancel the casting of the spell and had to resort to autoresolve to break the situation, which fortunately worked.