German Renaissance painter and printmaker Albrecht Dürer is possibly most famous today for his sketch of praying hands that was done as a study for a larger artwork. That larger artwork was a 1509 altarpiece titled Assumption of the Virgin, and it no longer exists (the image above is a 17th century copy by Jobst Harrich).
Dürer was commissioned to paint the altarpiece by Jacob Heller, but the agreed-upon price turned out to be insufficient, as the artwork was larger and more complex than Dürer estimated, and the price of paint and other materials was rising at an alarming rate. Heller added some to the price, but it wasn't enough to cover Dürer's expenses and he became increasingly angry about the project. Dürer got the last laugh, though, as he painted himself right in the middle of the altarpiece, right below the ascending Virgin Mary, in contemporary clothing, holding a sign with his name on it! Art historians have concluded this must have been an act of revenge. Dürer often placed his own image into paintings, but usually inconspicuously in a crowd in appropriate clothing. Read the account of the revenge of the starving artist at the Guardian. -via Damn Interesting
(Photo credit: Horst Ziegenfusz/Historisches Museum Frankfurt am Main)