The frame around colorbar (a thin black line) seems to be draw from lower left corner around all four edges, but without closing the line when coming back to lower left corner. This is only visible when thick linewidths are used or under a large magnification.

Yet worse, the frame around the plot itself is composed of four disjoint lines (it looks as if linecaps are "butt" rather than "round" or "square"), so it looks as if tiny little squares have been cut in all the four corners, like the lower left corner on the second image here:

http://studio.imagemagick.org/RMagick/doc/ex/rvg_linejoin.gif

(Image is taken from http://studio.imagemagick.org/RMagick/doc/rvgstyle.html)