Bones sometimes move/shift between edit mode and object mode. Even on armatures with absolutely no constraints or drivers. (And of course in rest pose, otherwise it would not be a bug ;-)).

See attached file for a simple example.

The subtle shift may seem insignificant, but this can become a huge problem for small characters, for example, or mechanical objects that require precision placement of joints. I am in fact currently working on a rig that requires such precise placement, and it is impossible for me to rig it correctly due to this.

I am running Linux Mint DE, 64 bit, with Blender SVN revision 37521.