This particular MESSENGER picture of Mercury's surface (explained here) is the first one I've seen in awhile that causes the "reversing crater" illusion rather easily, in which the craters suddenly optically transform into bumps. Some of the original images of Olympus Mons on Mars did this too -- the caldera always looked like a bump until better images came along.
If the craters look like bumps, the small crater at mid-left seems to revert back to a crater easier than the others. And that's the weird thing about this illusion; when one bump switches back to looking like a crater, all of them do. The mind is a wonderful thing, isn't it?
Here's a couple of mentions about the illusion:
Lunar craters inverting illusion
A lunar illusion you'll flip over
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