Today the user LUP amazingly denied an undo in the following circumstance: I had 9 (or 10?) coins, 1 province left, I accidentally clicked and bought a duchy. I mean, it is not even possible for this user to win at this point: I was 12 victory points ahead, no +1 buy, I drew 10 coins 80% of the time. It's more amusing than anything else how some people treat this game seriously. My rule of the thumb is...pretty much always allow undo. I have yet to see anyone abuse the undo so far.