In the Futurama film Bender's Big Score, the titular robot goes on a thieving spree through history, at the direction of three Scammer aliens stealing everything of value throughout all time. For his first theft, he wrestles the Mona Lisa from Leonardo da Vinci before the painting is finished, reportedly leaving the famed artist at least critically wounded.
This unfinished painting points up an aspect of historic treasures which should be recognized. The painting Bender stole is almost worthless--at most, worth considerably less than the version known to us. Before it was completed, it was just an unfinished canvas. It might have some value now as an unfinished masterpiece by a master--if the da Vinci who now is known only for The Last Supper (which contrary to the suggestion of Bender's quip was painted prior to this work) is famed enough, an unfinished painting would be of historical interest. Yet this still overlooks the principle aspect of what gives artistic masterpieces value. We value great works of art because we value them; we recognize within them something great, and so attribute to them great value in our desire to possess them. It would not be enough for the Mona Lisa to have been finished; it would also have to have been released to the world in some way, such that it could gain value by the desire of others to own it. A work that never met public eye would never have that kind of value; its value would be purely aesthetic (and our aliens show no appreciation for any kind of value that is not monetary) or limited to the value of a previously unknown painting by a known artist. The best the Scammers can hope is that they can proclaim it as the mysterious lost or stolen painting on which the great artist was working shortly before his death; even in this, we have to wonder whether if only one of da Vinci's two surviving paintings survived he would have been so well known, particularly as the other is part of a wall and cannot be moved.
Complicating this further, once Bender returns with the painting, having rendered it worthless and unknown in history he has eliminated any motivation he might have had for stealing it originally. He could not have known of its existence had it not become famous; he has prevented it from becoming famous. In so doing, he creates an(other) infinity loop, as history vacillates between the world in which Bender is motivated to steal the most famous painting which ever existed and the one in which it never did exist.
This complicates his task, in that he must wait for some objects to become famous and thus valuable before he can steal them. It does not apply to everything he takes--his theft of Egyptian artifacts will start our subject next time, but they have value partly from their materials and partly from their great antiquity. Yet this one theft underscores the complication facing many of the others, that of knowing when and where to steal an object for maximum value in the future. Take them too soon, and at least some will have less value than the unfinished Mona Lisa, and all reason for having taken them at all will be erased.