Simple enough to reproduce: take an army and move adjacent to an inn. Select a unit from the army which is not a Champion. Try to move onto the inn.

The unit gets split out of the army, but doesn't move. It's still visible in the square, but the army is "on top" of it so it can't be selected by clicking. I couldn't tab to it either but that could have been due to no moves left. Moving the army off the tile leaves that unit behind, at which point it can be re-merged or moved independently, so this isn't a killer bug but it's a little annoying.