I just had a game (3533161, decision ~200) where I Crowned a Sage and then my turn abruptly ended. Then my opponent bought the last Province very quickly and the game ended.
After careful examination of the log, here's what happened:
I had 1 debt; I had no treasures in hand
When I Crowned the second Sage, it drew two Duchies. Since I still had no money and 1 debt, my turn was over.
While this is correct, the user interface gave NO CLUE that this is what happened. To me it appeared that I clicked on Scheme to play it (twice) and then my turn and the game ended suddenly.
I suggest that there should always be an "End Turn" button, even if there's no other choice.