I've noticed that every once in a while, moving one of my units next to an opponent in tactical combat causes it to attack that opponent immediately.  The first few times, I thought maybe I'd clicked in the wrong place, but it's definitely happened when the square I wanted them to move to was highlighted (and the units always move to the target square, not where they would move automatically).  This is often harmless, but sometimes really annoying, e.g. because I wanted to use a spell or ability, or because I wanted to attack something else.