Short description of error
Exporting an animation to FBX produces artefacts when viewing the results either through FBX Review or when importing back into Blender
Exact steps for others to reproduce the error
Open the attached Blender file and hit play to observe the animation. Note how the feet are perfectly stationary while the character bends its knees.
Now export to FBX. In the FBX export panel, under animation, set Simplify to zero and uncheck the All Actions check box. Enter a filename and click on the Export FBX button.
Now either view with FBX Review or import back into Blender, set the end frame to 40 and play the animation. Observe how the feet are no longer stationary.
Below is the blender file for testing.