If you have both the Autogrid and PointGuide on, shift-selecting a point on an object made with a Shape Tool may reshape the object slightly. This happens because the PointGuide constrains to the nearest Autogrid point, which might not be exactly the point on which the object handle was located.
To see the problem:
1) Draw a star with the star shape tool
2) Turn on Autogrid and PointGuide
3) Select Reshape from the Arrange menu
4) Hold down the shift key and click one of the points of the shape
Workaround:
Turn off PointGuide or Autogrid. This will only happen when both of them are active.