If the player retreats from an enemy attack that was targetted at this sector, the enemy group pursuits instead of being given new orders. In the savegames posted about this error, this means that if one retreats to another town sector, the AI follows (with a 5-minute delay for internal reasons - if player and enemy groups arrive at the same time, bad things happen during tactical deployment).
Not sure how much harder this will make the game - I figure I'll just wait until someone comes crying and then make it optional (you wussies! ).