This example comes from Mike Lawrence. I'll include a link to his web site later, from which this comes and which contains great materials for B/I players, but for now consider this:
Partner leads the AK of ♠ at the first two tricks, and South follows suit both times (surprisingly ... partner bid spades twice on a five-bagger!).
Now partner leads the 6 of ♠ and declarer plays the 5 from dummy.
LOL, declarer played LOW from dummy ! The six is high, and you know declarer will have to ruff. So what is your play at trick 3 ? And why ??

Help

Partner opens 1♠, passed around to South, who overcalls 2♥. Partner bids 2♠ now, but North raises to 4♥, ending the auction.