In a post a couple of days ago I showed how easily the perception of colour can be shifted. The same applies the sense of lightness or darkness.
Here is one manipulated position of the ‘Koffka Ring’. What’s interesting here is that the grey areas marked A and B are, in fact, exactly the same shade of grey (RGB hex value #999999). But they sure don’t look like it.
Go here for an impressive flash animation of the Koffka Ring.