System Information
Windows 7 64, HD 5770

Blender Version
Broken: 2.71 release & e0d8e62

Short description of error
Extrude and Undo on a curves startpoint messes up the point order (numbering?) when Undo is executed in a different "Editmode session".
This negatively affects points that are assigned to a Hook.

Its complicated to describe, please take a look in the file.

Exact steps for others to reproduce the error

Setup for Hook:

1. Add a NurbsPath (Curve), add an Empty.
2. Give Curve a Hook Modifier.
3. Set Empty as Hook, and assign the middle control point of Curve.
4. Move Empty, Curve point should move aswell.

Bug:

1. Go into Curves Editmode.
2. Select the starting point and extrude one time.
3. Leave Editmode and enter again = different Editmode.
4. Undo your extruded starting point with STRG+Z.

Problem:
The assigned Hook point has changed. (move Empty)
It has shifted one point forward, related to extrude actions.