I have noticed recurring problems with calculating hp's and # of units for stacks. It seems to be a bigger issue if stacks are made and then the game is exited and re-loaded, and I haven't noticed a problem unless more than one stack of units is in the same city. Especially if one of the stacks loses a few guys. It's to the point where I can mouse over the same city three times and it will tell me different "hp/hp:", "hp/total_hp", and "# of units in each stack" each time I mouse-click.
I still haven't figured out if all units in a stack are supposed to "grow back" after losing some in battle or not because of this bug.
I haven't noticed this to be such a huge problem with normal dudes, but it happens almost universally with Darklings (Empire tree), and I recall losing a bunch of Umberloth's (or whatever the next tech in the Darkling tree is called) one time. Although I think it has happened with stacks of peasants and such, but I probably just wasn't paying that much attention because, really, who cares about peasants anyway?
-tid242