On several occassions I have had troops I KNEW were assigned to a stack go off course. I carefully watched one turn before going to the next and re-checked all move orders and sure enough I found the issue.
When you have a stack you are sending units to; my example (Eastern Depot) where I gather forces to be re-assigned as needed, you assign units to it but later in your turn you draw a unit from (Eastern Depot), lets call it (Western Reinforcements). Sometimes; (and here is where it gets tricky); sometimes the previously assigned units attach themselves to the unit(s) you drew from (Eastern Depot), so they attach to (Western Reinforcements). So instaed of the forces gathering at (Eastern Depot), they go all over the place to where you sent the units you broke off; in this case they go way west! Because the unit you assigned to (Eastern Depot) often cannot "catch up" to the broken off unit (Western Reinforcements) because problem two is it trys to go to region (Western Reinforcements) started but it ends up trying to short cut the distance between where (Western Reinforcements) started and where it is at the time. So the initial unit you assigned to (Eastern Depot) keeps following (Western Reinforcements) as far as it can and if they cant catch (Western Reinforcements) they just end up giving up and staying wherever it was when it lost (Western Reinforcements).
The only cure I find works for sure is to re-check all orders again before the end of the turn. Another problem is clutter if you broke of units they fill the region where they were broken off, this often covers the unit you broke them off from and makes re-dragging the little circles back to right unit "The easy fix", next to impossable.
Grrrrr. Opinions welcomed.