Dominion Last Province
You can always finish your turn. Whether you picked up that last Province in the action phase or the buy phase, you'll be able to keep playing until you're done with your turn, possibly picking up another victory card or two. Note that the only cards you can possibly find useful at this point are cards that give you victory points. (If you have Gardens, then you might buy any old cards just to make your deck bigger. Similarly, you might pick up Silvers if you have a Feodum.)