In the days when certificates were used it was common & quite easy. An instit. calls a broker & says sell 100,000 shs. of QEC. All securiies for that instit. were held by say Royal Trust & delivery against payment wud be settled by back office on settlement day. If there was no delivery no payment was made. When the back office at month end noticed the trade was still outstanding they tell the salesman who calls the a/c & the a/c says he will look into it. He might call back & say the securities were coming from Europe. They could normally stall for about 2 months. No one cared because no one had paid out money. The a/c can now buy back thru different broker & have the Royal Trust accept &deliver to cover short.

Even with present book based entries I think the same method could be used because the broker has no way to know if the instit. a/c actually owns the sold stock. The back office speaks to the Ry. Trust who in theory holds all the securities. They say they will check & get back & so on.

Regards

Joltin