I have a simple .gif image that I attempted to open and edit in GIMP 2.6.11 on Windows XP. The .gif image is quite simply a 4px by 170px top border for a box with rounded corners; that is, a long horizontal line with a curve at each end. On opening the image file, GIMP shows the horizontal line part, but the pixels relating to the curves at the extreme ends are missing.
This is the original .gif image:
This is the image having been opened using GIMP and then immediately saved back as a .gif image again:
I did a brief bit of Googling and did not find anything that seemed to relate to this problem. I did find some reference to a patent relating to the .gif format which meant that GIMP once didn't ship with .gif support; whether that's any relevance to this issue, or means that GIMP's .gif support will never be entirely complete, I don't know.
In the meantime I can simply use Windows Paint to do what I wanted to do with the image (which is to simply elongate it). I also use Ubuntu normally, so later on I'll boot back into that and see how GIMP behaves with this image there.
EDIT: I found that if I open the .gif image in Windows Paint and then save it, it will then open and render properly (i.e. with the curves) in GIMP.