After the AI has had its move and I'm about to have mine the UNDO button is active even though I've not moved or issued any orders. If I click this, which I've done in error a few times, it seems that this ends my turn completely, I cannot move attack or anything.
Now, I must qualify this post by saying that this is my first game, I'm still learning the interface and I could be wrong (but don't ever let my girlfriend hear that I've ever admitted I might be wrong
Other than that, this game is awesome!
