Housekeeping
By Housekeeping, I'm referring to all the tasks you have to take care of before you start cashing out top winners. Do you have to knock out an ace or a king or both? Do you have finesses to take. Suits to unblock? It may be losing a trick that you must do to position yourself for a squeeze. This is not to say you must not play top winners before doing all your housekeeping, for there are number of reasons why you'd want to do so. Nevertheless, I believe I can say Don't cash out top winners to which you always have access except for a positive reason. By "positive reason", I mean a reason why you feel this is necessary, a reason you'll feel comfortable offering your partner in the post-mortem.
I have referred to a failure to do this elsewhere in different wording, as when I've said, Don't uncover winners for the defense before necessary. But lately I've been coming across a number of instances where declarer's injured themselves by cashing out top winners before taking care of their chores, so to speak, and thus being set in a cold contract. Here are some illustrations.