<p>Instrument separation is a known endodontic treatment complication. In rare cases, broken instruments may end up in the maxillary sinus, complicating retrieval. Following a Peso reamer separation in the distobuccal root, a maxillary molar was placed into the sinus of a 20-year-old woman, and the instrument was intended to be retrieved from the root canal. However, in the next examination session, the Peso reamer extruded from the canal, moved apically, and resided in the maxillary sinus. Therefore, surgical intervention was necessary. The Caldwell-Luc approach was intended, and the instrument and extruded gutta-percha were removed. The root perforation was sealed with MTA, and endodontic retreatment was performed. The current case emphasizes the need for anatomical awareness and greater caution when handling instruments to reduce the risk of sinus or apical perforation. Furthermore, it highlights prompt intervention in managing iatrogenic complications related to the maxillary sinus to avoid instrument extrusion from the root canals and more complicated treatment.</p>