shorts have 3 days to cover. if they don't the stock goes on a threshold list and cannot be shorted until it comes off the list. the sec is lax about enforcing this. but the brokerages keep track and when they get a hint that there are not enough shares to cover the shorts, ( it has happened ) they will no longer accept buy orders on the stock. this happened in cprk ( apparently in the last week.) you can get a lot of info about shorting from that board if you want to take the time to read through a jillion postings.