Wearing metal accesories like rings, piercings and earing during a surgical procedure at the surgical room can make interference with the electrocautery, and it can cause undesirable burns in the transsurgical period. Nylon and Lycra underwear also can cause burns. That's why you are not supposed to wear them in the surgical room.
In general, piercings are metal, and they conduct heat. In surgery, many times we use a cautery and this conducts heat which can then burn the skin over the metal piercings.