These are the steps I have taken

1. I create a polygon cube 150x15x150 like the tutorial



2. Insert two edge loops using the edge loop tool



3. Select the top right corner vertices by dragging over them in the top down view
4. hold V and middle click to drag to the vertices on the left.

This normally works as planned

5. the black faces usually appear when I attempt to drag the vertices down to complete the corner cut out of the room.