Asymmetric risk: profit is capped at 100% (the price cannot fall below zero), while losses are unbounded.
The frictions that matter and get overlooked in interviews: borrow must be located and can be expensive or unavailable; the lender can recall it, forcing you to close at the worst moment; you pay any dividends to the lender; and margin requirements can force liquidation.
Why this breaks textbook arguments. Many no-arbitrage relationships assume free unlimited shorting. When borrow is expensive, apparent mispricings can persist indefinitely.