Not an urban myth, it seems.
One of the gems from Greg Smith's Goldman Lovefest Why I Left Goldman Sachs, in which he tells of the origin of firm's intern elevator rule.
It all started a few years back when Josh, an intern and 'son of a billionaire', 'committed a gaffe that resulted in a rule that to this day is laid down to all Goldman summer interns: When you're in (a firm) elevator, do not say anything. It doesn't matter what - a joke or even a remark about the weather - just keep your mouth zipped. You simply never know who might be in the elevator with you.
Josh learned this the hard way. He was riding up in a full elevator on a day when the head of the Chicago office was coming to speak to all interns, and when someone mentioned the event, Josh chimed in, 'The head of the Chicago office ? Who the fuck cares about the head of the Chicago office ?' You guessed it: the head of the Chicago office was standing at the front of the crowded elevator'.
Old Josh apparently never did get a permanent job at Goldman.