Suppose that you are comparing put and call prices on the same underlying stock and the strike prices and time-to-expiration of the two options match. Further suppose that there are no dividends expected for the coming year on the stock and the options are all European. If the put-call parity shows that the market price of the put is greater than the synthetic put, you could have an arbitrage profit if you:
buy the put , buy the call, sell stock, sell a bond
buy the stock, buy the bond, write the put, write the call
buy a put, buy stock, write the call, sell bond
no arbitrage is available for these asset prices
buy the call, buy a bond, write the put, sell stock