When in the heat of trading 20 different stocks and handling millions of shares of order flow, it was quite common to short against the order flow without giving a lot of thought to it.
But, and this is important to understand, the firm who is backing your action will approach you with a BUY-IN notice if shares cannot be found to cover your short.
They typically give you 1 to 2 weeks to cover or have a FORCED buy-in.
That's right, they go in the open market and buy in your uncovered short and charge it to your account.
It happened to me on numerous occasions. I ALWAYS bought it in myself to avoid getting crucified by them. Sometimes with a 1 day or same day (cash) settlement.
Bottom line...the naked shorts have to cover at some point. That I can guarantee you. No firm is going to risk scandal and BK by NOT covering at crunch time.