3-4 AI armies came at one of my towns, so i brought my defenders out of the city to meet them before they could sack improvements.  I killed 2, ran out of move, and ended turn.  The remaining army moved right over my army and attacked the town.  I've seen the AI move over my units before, but never when we were at war.

They didn't move around, as pathing around would have taken 4 move, the army only had 2.

Edit: on reload of the auto-save, they attempt to move around, so it doesn't appear to be repeatable.