In most cases, a sale is only "short" if the seller cannot afford to make up the difference in what they're selling it for and what they owe to close out the existing mortgage. Obviously if you will owe money but have the money to pay, it's not an issue.
What makes these complicated in a lot of cases is banks have to decide that they are willing to close out the loan for less than they are owed. Since, in many cases, the interest over the life of the loan has still allowed the bank to earn a profit on the loan it is simply a case of the seller going through the steps to get bank approval. The last thing a bank wants to do is own property, especially these days.
As a buyer, it's important for you (and your agent) to look at the loans on record at the registry of deeds to make sure you know the financial situation of the seller. A short sale can delay the process and many times rate-locks can be at risk or a house/apartment you are leaving needs to be vacated before the process can be completed at the bank's end.
Finally, inspections are CRITICAL on potential short sales because anyone who has not been able to stay ahead on their mortgage might have needed to cut corners on long-term maintenance.