Unpredictable animation - vertex keys

Hi there! I am sorry to have to ask what probably is yet another naive question, but I have not managed to sort this one out. I have been experimenting with shape keys. The following .blend contains a “basis” shape (a plane) and two differnt shape keys.


Next I created two shape keys in the action editor, and created actions for them. I imported them into the NLA editor and converted them into NLA strips. I pasted the strips in sequence, so the shape blends into the two actions in sequence.

Where’s the snag? It’s that if I, for instance, play the animation (Alt-A) and stop it (pressing Esc.,for e.g.), dragging the slider to the zero-frame does not always give the basis shape as expected. Why would this be so? is there anything I can do to ensure that the shape of the mesh is predictable on frame zero? I expect to see the basis shape (the undistorted plane) but the remnants of the shape keys often unexplicably persist.
Many thanks,