Every now and then, a surgeon sews a patient up with a surprise left inside. Scissors, sponges, and so on eventually make their presences known, but if you don't relish feeling like the patient in the game of Operation, your surgeon can purchase surgical sponges that come with an RFID chip sewn in.
At the beginning of the procedure, the surgeon swipes the packet of sponges over a sensor that registers the contents. Then, as she uses them up, she drops them into a pail equipped with a sensor that counts them as they fall in. If any are AWOL at the end of the procedure, she can wave a sensor over the patient's body to see if any of them were, ah, left behind.