This case study illustrates a rare complication of methemoglobinemia secondary to Patent Blue dye injection during bilateral skin sparing mastectomy with bilateral sentinel lymph node biopsy with immediate free flap (DIEP) reconstruction in a 49-year-old female with right-sided breast cancer. Following the administration of patent blue dye, the anaesthetist detected falling oxygen saturation to 90% and an arterial blood gas confirmed a peak methemoglobinemia level of 8.7%. This equates to a 10% reduction in the patient’s haemaglobin oxygen carrying potential. Prompt recognition of the methemoglobinemia's severity and its potential to compromise oxygen delivery prompted immediate intervention. The patient responded to conservative management with 100% oxygen therapy, a cornerstone in the treatment of methemoglobinemia, and required intensive care unit (ICU) admission for close monitoring and supportive care. This case sheds light on the significance of meticulous vigilance in the use of patent blue dye, a commonly employed agent in lymphatic mapping in cancer surgery. The unexpected development of methemoglobinemia in this context underscores the need for heightened awareness among surgical teams regarding the potential adverse effects associated with intraoperative dyes. Furthermore, the successful management of this rare complication with prompt recognition and appropriate intervention highlights the importance of interdisciplinary collaboration and expertise in navigating unforeseen challenges in complex surgical procedures.