Just had a WTF moment when Relias finished construction of the Great Mill on turn 70 or something like that. I suspected that the trigger was loading a savegame (yeah, yeah, I made a blunder in tac combat - stupid clumsy trait!), and went on to test it.
Sure enough, every time you load a game and the AI has improvements on its queue, they *ALL* get instantly completed the next turn (needless to say, it costs no gildar, not even the 25 minimum). This happens even when the front of the queue is a unit - all the improvements next in queue get completed (the unit itself doesn't, though - the bug seems to pertain to improvements only).
I suppose this could be Brad's cruel punishment for savescumming
I'm aware that there's a new release, but I haven't seen it addressed in the changelog so it's probably still there...
Also, for the record, I was playing on challenging, but I don't see any particular reason it shouldn't happen on other difficulty levels.
edit: verified it's still there in 1.004