In any case, the important limit here is that financial firms are not supposed to allow hedge funds or other entities to assume short positions for more stock than exists, because if it becomes necessary to execute the trades to resolve the shorts, that extra 40% will fail to deliver, because those shares don't exist.
The SEC actually keeps a list of trading companies with high rates of failure to deliver as a means of detecting naked shorting.
In any case, the important limit here is that financial firms are not supposed to allow hedge funds or other entities to assume short positions for more stock than exists, because if it becomes necessary to execute the trades to resolve the shorts, that extra 40% will fail to deliver, because those shares don't exist.
The SEC actually keeps a list of trading companies with high rates of failure to deliver as a means of detecting naked shorting.