Im using Gimp 2.8.6 on Ubuntu 13.10 (64-bit)
Not all the time, but randomly, Gimp chooses not to
stroke a path correctly. Here is a "before and after stroke" picture (it's a close-up) :
What is wrong with my Gimp / options? Why does this happen?
It is very hard to tell without having it installed and playing around, but what I see is a horizontal line with an unconnected vertical line below it with 90 degree turns at the ends (zoom in on your screen-cap and look at the faint pink line under the dot). Try joining the paths or altering the endpoint types.
So you have a 6px stroke on a path with two turns which each have a sub-pixel unit length, an open-ended cap style, and a 10px mitre limit on a vertical path that looks to be approx 12px long. That's a lot going on in a tight space.
The "turns" could just be some oddity with the rendering of the path lines, I don't know for sure.