In-game map isn't rendered on the edges of the screen and you can see the wall behind it instead. This happens only when you look at the map at some angles though.
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Actually the maps were explored properly (the fragment above T-shaped lake is visible on the second screenshot). The bug might be MC-1058, but it wasn't visible to me in 13w39b, so something with rendering the map might have changed between two versions.
...because you didn't explore the map properly, at least it looks like it for me.