Code violations abound. Which is what all of the above are.To answer your question in more detail....
Yes, it is all too commonly done. If you hook the neutral and ground together, you MUST be certain that the outlet is wired correctly, polarity wise, or bad things will happen when you plug in your pigtail.
A common trick, is to install a three prong, grounded outlet recepticle, putting a jumper from the neutral to the ground on the outlet itself.
Another common trick, is to hook the neutral and ground together in the furnace, and physically clamp a ground wire to the gas line.
I am not advocating any of the above. I am describing them for you, so that if/when you come across these in the field, you will know what you are looking at.