It looks like some one has run their fingers across this frame at the National Gallery, breaking off a big chunk of gilding. I assume it happened quite recently, as the fragment is still there. There are quite a few other chips where the gesso is bright white, indicating that they could be recent. The picture, Departure of the Argonauts by the Master of 1487, is on loan from a private collector. It's in the same gallery as the celebrated Botticelli Mars and Venus and it's often packed, but the guards has to cover the room with the Leonardo as well, so it's usually unattended. Two renaissance cassoni are used as tripods for photos, and the gilding is noticeably worn in the middle.
Cost cutting in museums imposes real costs, alas.