When shares are scarce, borrow rates rise from near zero to double digits annually. That cost has to appear somewhere, and it appears in the options: puts get expensive relative to calls, and put-call-parity appears to break until you include the borrow.
Two practical consequences. First, a synthetic-short-stock in a hard-to-borrow name is not free money — the cost is baked into the quotes. Second, an assignment that leaves you short shares can trigger a forced buy-in at any time.
Example: XYZ at $50 with a 40% borrow rate. The 90-day $50 put trades $2.10 above the call rather than at parity. That $2.10 is roughly 40% annualised on $50 for 90 days — the borrow cost, expressed as an option price.
Related: synthetic-short-stock, reversal-arbitrage, short-selling, put-call-parity