It's a dirty little secret... we beta testers did not manage to convince the developers that the autosaves are wonky, despite multiple threads on the subject.
So here is a piece of advice from someone who has been playing since April.
Do your own saves, at the END of every turn, that is, right before you press [End turn]. A save made at any other time is suspect. Autosaves... well, autosaves can be anything. At the very least, you will lose a turn worth of mana/production/research/etc.. At the very worst, you may lose whole units and building that were just about to finish, hundreds of mana worth of enchantments, and some of your units will get caught my monsters that would otherwise be out of range.
