The Night Watch
The painting you see today is smaller than the one Rembrandt made, because somebody took a knife to it to make it fit a wall.
Rembrandt's enormous group portrait of a militia company was never meant to be called The Night Watch. That nickname came later, when layers of darkened varnish made the scene look like it took place at night. It is actually broad daylight. The real title is something closer to The Militia Company of District II under the Command of Captain Frans Banninck Cocq.
In 1715 the canvas was moved to Amsterdam's town hall and, to fit between two doors, it was simply trimmed down on all four sides. The largest cut came off the left, removing two figures and throwing off Rembrandt's careful composition. The original dimensions survive only in an old copy. The painting you admire now is a cropped version of a lost original.
It has also been attacked three times. In 1911 a man slashed at it with a shoemaker's knife. In 1975 an unemployed teacher cut deep zigzag gashes into the canvas. And in 1990 a visitor sprayed it with acid, though guards rinsed it with water before the acid reached the paint layer. The wounds were repaired, but if you know where to look, the restorations are part of its history now.
See it now: Notice how Captain Cocq stands oddly close to the left edge, with space stolen from him by the 1715 cut, and look for the bright little girl glowing in the crowd like she is lit from inside.