1. Create cube
2. Activate new material
3. Uncheck Keep Material check box when selecting rectangle tool
4. Draw rectangle 3D extrusion option on cube to cut part of it.
5. New cut faces should have different material than rest of the cube but they are not.
6. Check Keep Material box
7. Draw rectangle cut option on again: now previous cut faces will turn to different material.
When you are cutting more complex object with this method the trick 7. won't even work.