Dental practitioners frequently encounter with instrument fracture during root canal treatment. However, these fractures generally occur inside the root canals. It is not usual that high speed rotary instruments snap and go further to important anatomical landmarks. In our case report, a high-speed Gates-Glidden drill perforated the root and was introduced into the left maxillary sinus during a routine root canal treatment. The dental and surgical management of the case are described and the possibly more serious sequelae are discussed. Practitioners must take great care during dental treatment and endodontic treatment in particular, not to accidentally introduce foreign bodies into maxillary antrum.