From http://bugs.debian.org/577623:

When converting an RGB image to indexed, the GIMP sometimes fails to
choose as accurately as it could between similar colors in a chosen
palette, even when each pixel has an exact match somewhere in the
palette.

The enclosed colormaptest.ppm and Colormap_test files together form a
test case to reproduce this bug, as follows:

1. Compare colormaptest.ppm (a text PPM image) and Colormap_test (a GIMP
   palette) to verify that each contains the same RGB triples in the
   same order.
2. Move or copy Colormap_test into a directory for GIMP palettes, e.g.,
   the ~/.gimp-2.6/palettes/ directory.
3. Start the GIMP and open colormaptest.ppm, then zoom in enough that
   you can see each individual pixel.
4. Right-click image -> Image -> Mode -> Indexed... to open the Indexed
   Color Conversion dialogue, and set it as follows:
   a. Under "Colormap", select "Use custom palette", then click the
      button below that selection to pop up the list of 
      available palettes.  From this list select "Colormap_test".
      Uncheck the "Remove unused colors from colormap" checkbox if it's
      checked.
   b. Under "Dithering", for "Color dithering:" select "None".  Uncheck
      the "Enable dithering of transparency" checkbox if it's checked.
   c. Click "Convert".
5. Observe that while the leftmost and middle pixels have been mapped to
   their exact matches in the palette, the rightmost pixel has been
   mapped to the same color as the middle pixel.  (Try pressing ^Z to
   undo and ^Y to redo repeatedly if you don't see it at first.)