Problem:
Taro and Hanako have numbers of cards in their hands. Each of the cards has a score on it. Taro and Hanako wish to make the total scores of their cards equal by exchanging one card in one's hand with one card in the other's hand. Which of the cards should be exchanged with which?  Note that they have to exchange their cards even if they already have cards of the same total score.

Incorrect submission (verdict: Wrong Answer):
def impossible(taro_cards, hanako_cards, average):
    if not isinstance(average, int):
        return True
    elif sum(taro_cards) + max(taro_cards) - min(hanako_cards) < average:
        return True
    elif sum(hanako_cards) + max(hanako_cards) - min(taro_cards) < average:
        return True
    else:
        return False


while True:
    n_taro, n_hanako = map(int, input().split())
    if n_taro == 0 and n_hanako == 0:
        break

    taro_cards = []
    hanako_cards = []
    change_pairs = []

    for i in range(n_taro):
        taro_cards.append(int(input()))
    for i in range(n_hanako):
        hanako_cards.append(int(input()))

    average = (sum(taro_cards) + sum(hanako_cards)) / 2

    if impossible(taro_cards, hanako_cards, average):
        print(-1)
    else:
        # record pairs of cards to be changed
        for i in range(len(taro_cards)):
            for j in range(len(hanako_cards)):
                if sum(taro_cards[:i]) + sum(taro_cards[i+1:]) + hanako_cards[j] == average:
                    change_pairs.append((taro_cards[i], hanako_cards[j]))
                    # print("{0} {1}".format(taro_cards[i], hanako_cards[j]))

        # print the pair of cards to be changed
        pairs_leastsum = change_pairs[0]
        for i in range(1, len(change_pairs)):
            if sum(change_pairs[i]) <= sum(pairs_leastsum):
                pairs_leastsum = change_pairs[i]
        print(*pairs_leastsum)

Fixed, accepted submission:
def impossible(taro_cards, hanako_cards, average):
    if (sum(taro_cards) + sum(hanako_cards)) % 2 == 1:
        return True
    elif sum(taro_cards) + max(taro_cards) - min(hanako_cards) < average:
        return True
    elif sum(hanako_cards) + max(hanako_cards) - min(taro_cards) < average:
        return True
    else:
        return False


while True:
    n_taro, n_hanako = map(int, input().split())
    if n_taro == 0 and n_hanako == 0:
        break

    taro_cards = []
    hanako_cards = []
    change_pairs = []

    for _ in range(n_taro):
        taro_cards.append(int(input()))
    for _ in range(n_hanako):
        hanako_cards.append(int(input()))

    average = (sum(taro_cards) + sum(hanako_cards)) // 2

    if impossible(taro_cards, hanako_cards, average):
        print(-1)
    else:
        # record pairs of cards to be changed
        for i in range(len(taro_cards)):
            for j in range(len(hanako_cards)):
                if sum(taro_cards[:i]) + sum(taro_cards[i+1:]) + hanako_cards[j] == average:
                    change_pairs.append((taro_cards[i], hanako_cards[j]))
                    # print("{0} {1}".format(taro_cards[i], hanako_cards[j]))

        # print the pair of cards to be changed
        if change_pairs == []:
            print(-1)
        else:
            pairs_leastsum = change_pairs[0]
            for i in range(1, len(change_pairs)):
                if sum(change_pairs[i]) <= sum(pairs_leastsum):
                    pairs_leastsum = change_pairs[i]
            print(*pairs_leastsum)