Heh, I'd like to add the annoying side of this bug too:

If you are besieged, and you try to retrain a few units which have lost a couple of men, then get attacked and loose a few more men.. the turn after the siege is over(assuming you survived the assault ), your units have retrained, but not to full strength! They miss the amount they lost in the recent battle. While if you recall the retrain just after the siege, and click retrain again... they WILL get full strength the turn after.