Windows 7 Professional 64-bit w Service Pack 1
Display card: Nvidia GeForce GTX 760
Blender 2.69 - r60995
There is a bug editing aligned handles of an animation curve.
See attached file: “handle editing bug.blend”
In the curve editor, select the right hand handle of the middle keyframe as in first image, “handle edit A.jpg”.
Now move press 'G' and move the handle to the position as indicated in the second image file, “handle edit B.jpg”
It will be seen that the behaviour of the handle on the opposite side of the keyframe anchor point is not moving correctly. It is as if the control point of the second handle wants to remain vertically locked to the frame it is on. This behviour is not correct and often leads to a wild overshoot of the bezier curve.
The correct behaviour of the handle on the opposite side when being edited is indicated in the last image, “handle edit C.jpg”. Here we see that the handle on the opposite side remains the same length (thus retaining the same 'tension') and simply rotates around the keyframe anchor point.