I was fascinated to read how, while cleaning a Dutch painting, an art student at the Hamilton Kerr Institute in England discovered that it had been altered in the past, obscuring its central feature - a stranded whale - without which the painting made very little sense.
Here's a video report on how the discovery was made. I recommend watching it in full-screen mode.
I suppose, after the passage of so much time, we'll never learn for sure who covered up the whale, or why . . .