Quote from ptrjon:
to answer the fundamental question from the OP, let's say there's a company with 100 total shares in the market. This company pays $100 in dividends a year, $1 to each share.
Suzy short thinks the company will lose value so she borrows 10 shares from another holder and sells them to a new shareholder. Now, there are 110 long sharess, and 10 short shares.
So when dividends get paid, the corporation pays $100, while suzy pays $10 to the person she has traded against. So yes, there are 10 "fake" shares.
As far as bankruptcy, etc- this is a non-concern. The brokerage that holds the short position monitors Suzy's account to make sure that she's able to cover the loss, and then some, at all times. If there's any question, they will force her to buy to close. If it's too late and Suzy has lost more than she can afford, the brokerage is on the hook. The brokerage must cover all the losses, and will probably buy to close the position immediately. If this causes too many losses for the brokerage that they go bankrupt, the SIPC steps in, covers losses, and helps clients recover their investments.
Fun stuff, huh.