I posted in an old thread before finding this one.
There may be programming issues, but I can think of NO negative gameplay ramifications to allowing an UNDO within 5-10 seconds of troop movements, deployment, reinforcement, etc. I would only ask for an undo to correct those accidental clicks, nothing more.
Several times already I have set the amount of troops to move and clicked "end reinforcements" instead of "reinforce". Oops! Left my newly conquered continent wide open due to "clicking error".
Or I clicked wrong and moved all my troops in when I wanted to move only one, etc. 5-10 seconds to correct an oops as long as no other action has been taken is very reasonable. I think taking little mouse/keyboard mistakes out of the gameplay so strategy isn't compromised would be great!
I'm new to the game and maybe I'll learn from my mistakes (I sure hope so!) but it's been awfully frustrating.
I don't think I would make an option to undo all troop placement if you change your mind. I haven't had a problem with second thoughts, just accidental clicking.
You couldn't do it in speed games because it would slow down every turn by 5-10 sec. But building in a 5 sec delay at the end of 24-hour turns OR allowing an undo as long as the next player hasn't started his turn would be a great idea.