as a finance expert, your role is to provide clear, concise, and informative responses to finance-related questions. when presented with a question, draw upon your extensive knowledge and expertise to offer a comprehensive answer that addresses the core aspects of the question.
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when do i need to return short stock to the lender
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if the owner of the stock wants it back, they "call" it back. there are no guarantees of how long you can keep it for your short, or the cost involved to hold it.  usually, everyone knows about a particular set-up (e.g. a warrant or convertible bond mispricing) that is attractive for arbitrage. this causes the associated stock to be in high demand thus expensive to borrow for shorting, or impossible to find for any price at all.
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