Sometimes museums get in trouble. Deep trouble. Not because they damage art, or let it get stolen ... but because they sell it. The Delaware Art Museum is the latest target of the art world's ire — for selling one painting from its collection to try and tackle a debt, and for revelations in the past few days that two more paintings are up for sale.

The controversy relates to a serious museum practice with an unfriendly name: "deaccessioning," or the permanent removal of an object from the collection. There are rules around when and how deaccessioning can take place. Break those rules and there are some unpleasant consequences.