Windows 7 x64
NVIDIA GeForce GTX 580
Broken: 2.78a e8299c8
Short description of error
The Pose Breakdowner tool doesn't work when in Tweak Mode on an NLA Strip whose Start Frame doesn't coincide with its Action's Start Frame.
Exact steps for others to reproduce the error
Open new .blend file, open a Dope Sheet editor and switch to the Action Editor context, and open an NLA Editor. Create an Armature, switch to Pose Mode and create a few keyframes. Select all the keyframes in the Action Editor and switch their Interpolation Mode to Constant. Push Down the Action into the NLA Editor as a new Strip. Grab the Strip in the NLA Editor and translate it along X. Enter Tweak Mode on the Strip, place the playhead between two keyframes and call the Pose Breakdowner.
In the attached .blend, the ArmatureAction Strip behaves as expected and the ArmatureAction.001 Strip exhibits the problem.
Expect this has to do with the discrepancy between the Action and the NLA Strip's Start Frames. An easy (?) fix might be to have the Pose Breakdowner compensate by offsetting itself to the Strip's Start Frame when it recognizes that it's been called during Tweak Mode.