owner owing difference?

[Mathew Wray]

I'm curious about something I've heard repeated a few times. When a short sale goes to closing is the owner expected to come up with the difference between the sale price and the mortgage price? I've been told that one of the difficulties in dealing with short sales is that often the sellers don't know, or don't want to know, that they have to pony up the difference in the sale price and the mortgage price so when they find that out at closing they often end up walking away and just letting their property get foreclosed on.

Is this common or just a risk or am I way off base?

Thanks!

Mathew