In Sans at 14px the dot over «ż» is shifted too far to the left, as rendered by freetype with bytecode hinting enabled. In Sans Mono «z» looks the same but the dot is moved 2 pixels to the right, which is perfect (the width of the «z» is even, and the dot looks better in the right one of the two center pixels).
Also, the dot should look the same as the dot over «i», which is 1px×2px at this size. At 15px the dot over «ż» changes to 2px×2px, while the dot over «i» is still 1px×2px. This applies to all DejaVu fonts. 1px×2px at these sizes should be fine. See http://www.twardoch.com/download/polishhowto/kropka.html
I think this bug is same as https://bugs.freedesktop.org/show_bug.cgi?id=7224 . Instructions of accented glyphs do not properly set the positions of accents.
Indeed, it's the same as bug 7224. Marking as duplicate. If there are more letters with misplaced accents in an apparent way, you can leave a comment there.
Thanks for reporting
*** This bug has been marked as a duplicate of bug 7224 ***