elementaryOS 0.4.1, GTX1080
Broken: 2.79.6, 104fb1f
Short description of error
Enabling the Motion Blur option of the Mask node in Compositor overrides the Feather Falloff type, which you can set per MaskLayer in Image / Clip Editor. With Motion Blur enabled, the falloff is always the default type ('SMOOTH').
Exact steps for others to reproduce the error
Open this blendfile
In the Mask node enable Motion Blur, then disable it again. Note how the mask's feather falloff looks totally different, even though shutter speed is set to 0.0001. I suspect that in COM_MaskNode.cpp the feather falloff is somehow ignored when MASK_MOTION_BLUR is enabled. Or something like that. I should learn C++ ...