So we know about the computer's ability to suddenly pull out a full-stack army or give itself money even when you're blockading ports.
I just found that they can sally out more than once a turn, too.
Quintus Scipio garrisoned an army in the city south of Carthage. I'd had my heart set on taking the whole of Africa for a while, so I lay siege. He sallys out before I have any siege equipment built, but stays within the walls of the city. Okay - I've never seen that, but I use the opportunity to just cut down his ranks with archers until I notice there's a hole way out on the far right side of the settlement. With about half the mission time, I set my guys on the way, start kicking ass and taking names, and before I can take the city, time runs out. A draw.
(Side note - that's bogus.)
Fair enough. I cut down his ranks, had most of my army left, etc.
Before the Scipii turn is over, he sallies out again. Same city. Hand on the Bible.