"Virgin of the Rocks"
The first, painted 1483-1486, the second painted from 1495-1508. Da Vinci, ever the perfectionist, fiddled with his paintings for quite a long time, to the exasperation of his clients. Lots of theories exist to answer the question: "Why two paintings?" Finding the sometimes subtle difference between the two can take longer than you may think.
Remind you of this?