I've noticed this happen before.  It is easy to reproduce if you make an attack, then go to the board explorer or history or otherwise leave the page.

This just happened to me during some lag, I attacked and the board didn't update showing the attack had taken place.  The spot I attacked from when viewing shows the original number of units, lets call it x.  But when I click the spot to attack from it the unit count is updated to x-3.  And clicking the spot I attacked updates from "fogged" to 3.

So the attack took place but the transfer screen never popped up.

Edit: Oddly enough, loading the player from another page brought up the transfer units pop-up.  In the past when this popup was already up and I left the screen it would not reappear.

Turn 957

http://www.wargear.net/games/view/475295

I have this lag issue too - but usually I just hit the reload button (on the board) and it brings me back to the correct place. 

Would be awesome if it got fixed.