Short description of error
"Follow curve" option in follow path constraint makes constrained object flip around if first (or second) vertex of curve is manipulated
Exact steps for others to reproduce the error
Move around any of the two bones I point at in the blend, it makes the constrained bone flip around. As I pointed out in the blend also, checking "follow curve" makes the constrained object point in another direction (which doesn't seem to make sense, as it seemed correct *before*) - so a readjust of the "up" and "forward" axes is needed afterward.