Summary: in the fortification phase of a turn, if the player sets the pop-up menus to non-default values and then clicks "End Fortification", put up an "Are you sure?" dialog.
Description: I've played a few games and made one particular mistake several times already. I don't think I'm alone in making it - I'm playing with a few friends and they've complained about doing the same thing. That is, after finishing attacks for a turn, you carefully set up a fortification of n armies from some place to another place - and then spoil it all by pressing the wrong "End Fortification" button instead of "Fortify" thus leaving your armies in the wrong place.
As I see it there are two problems with the UI at the moment: firstly, from the perspective of being a Mac user, all the "OK" buttons are consistently on the right side of dialog boxes; this means I have to overcome my muscle memory in order to hit the correct button. Secondly, it is not clear from the title of the button alone, exactly what its effect is; ending a phase neither implies committing nor cancelling user interface changes made in the meantime.
There are two changes you could make to the site which would mitigate this.
The easiest would be to retitle the "End Fortification" button to something more explicit, such as "Cancel Fortification" or "No Fortification".
A more involved (but more reliable) change would be to add a "safety catch" to the End Fortification button: if (and only if) the user has changed some of the pop-up menus away from their default values, it is quite likely they intend to fortify. Thus in those circumstances if they press the "End Fortification" button, it would be reasonable to ask for confirmation with an additional "Are You Sure?" box, since the chances are that they have made a mistake.
[Edited: correct a typo]