I knocked off a yellow army, and was surprised when another army was in its place; obviously more than one army can occupy a space, but the number over their heads had blended in to the background.  When I looked closer, I realized there were 18 armies on one spot.

All turned out well; I turned, ran, and drew them into the zone of control of a well-upgraded outpost on a coastline.  Tidal Wave is a nice spell.

I suppose this could include your own armies, but I'm thinking in terms of the computer.  Also, I was playing on easy, which may explain why the computer did something so silly.