The British episode known as "Mary's Mistake". An elderly woman accidentally says No Deal, gets a chance to recant the decision, says No Deal anyway, and proceeds to win the largest amount left in the game (£75,000) after doing a perfect round.
Pretty much any other time someone manages to go home with one of the Power Five after a bad start.
Suzanne Mulholland, whose final four boxes were all from the Power Five. She then proceeded to knock out the two smallest amounts (£35,000 and £50,000) in order, leaving her with the "Dream Finish" of £100,000 and £250,000 — the first time someone managed to do this in the UK version — and a new record set for the highest offered Banker deal (£165,000). And then she swaps boxes (something no one else with the £250,000 in one of the final two boxes has done before) and wins the quarter million!
December 2009, again in Britain: Neil's game, in which he, despite having the top four amounts still in play at 8-box, and having 6 red boxes at this point, deals for £35,000, explaining that the number selections that were guiding him through the game were no longer occurring to him. He then proceeds to knock out all four of the top amounts, in sequence, with his next four picks, the highest possible amount of winnable money now being £20,000. After the end of the game, Noel told to the pilgrims who thought dealing was a mistake to stand, and say "Sorry Neil".
The first Australian win was pretty awesome, too, for different reasons: the contestant was left with no safety net after he got down to five suitcases with the two highest amounts left in play being $200,000 and $7,500, which made the audience (and the host) nervous enough to favor him dealing in a vote. He still said "no deal" and got down to two suitcases with $200,000 and $5 in play and a very tempting bank offer of $102,500, and the remaining suitcase-holding contestant said herself that she would have dealt at this point. He said no deal, opened his suitcase, and the rest is history. The comments to this video all agree that he must have had huge balls to do what he did.
This one crosses over with Heartwarming Moments: On the American version, a contestant named Pyong was playing for the million. At one point, he mentioned that his parents, who were in the audience, immigrated to the United States from Korea with $750 in a suitcase. In one of the greatest cases of There Are No Coincidences ever, his final three cases left when he accepted the Banker's offer were $75, $750, and $750,000. When it came time to reveal what his case was worth...well, the reaction says it all.