Giotto

This used to be shown as the middle piece of a bigger altarpiece — five panels, according to the National Gallery — until they got separated somewhere along the way. It was even catalogued as someone else's work for a while before curators settled on who actually painted it.

This one's part of a set — seven small panels showing scenes from the life of Christ, according to the Met, though nobody's fully sure which church they were first painted for.