Windows 7 Ultimate x64. Titan X graphic card.
Broken: 2.79a, mask on sculpt mode
Short description of error
Blender crashes when you invert the mask areas and "undo" the changes you did into the last masked area.
Exact steps for others to reproduce the error
Make a cube, sculpt anything. Press "M" to go into mask mode, paint an area. Invert the mask
Sculpt or modify anything inside the mask, Invert the mode (alt+M) and click CTRL+Z. Blender crashes.