Dummy Reversal
Some hands count out easily to winners, some to a count of losers from either hand and some most definitely count out far easier from one hand or the other. In such hands, the closed hand is without question the most common one to count losers from, being that this is commonly the hand with the long trump holding. The problem arises when dummy clearly is easier to count from, for some declarers are so locked into counting from the closed hand that they cannot see how much easier -- and more productive -- it would be to count from dummy. Indeed, in an ironic twist, there are players given to transfer bids specifically designed to put the long trump holding in dummy who still cannot escape thinking they want to ruff in dummy if possible.
Below are links to a variety of illustrations of such failures to work out a hand from dummy. The first one here was such a naked display of this proclivity that it was the one that induced me to include this as a common reason for going down unnecessarily.