Purpose: Direct neurotization of free flaps is an emerging microsurgical technique used to improve functional outcomes following reconstructive surgery. In contrast, unintentional reinnervation of a free flap to a donor nerve in an oncological reception bed may lead to poor functional outcomes. This case reports the reinnervation of an insensate fibular free flap by the glossopharyngeal nerve, leading to disabling gag reflex activation on light touch. Methodology: A 61-year-old man presented to a major head and neck cancer service for work-up and management of a left retromolar trigone (RMT) squamous cell carcinoma. He proceeded to wide local excision, partial pharyngectomy, segmental mandibulectomy, left level I-IV neck dissection, and right fibular free flap reconstruction. Five months post treatment, he presented with distressing gag reflex upon touching his fibular free flap skin paddle, impairing oral intake. Results: Examination confirmed a reproducible gag reflex on light touch to the flap inset on the left buccal surface. Application of topical anaesthetic to the flap provided rapid and effective relief. Conclusion: This case demonstrates a potentially preventable and rare complication of innervated flaps. Segregation of recipient site and donor site nerve fibres may prevent aberrant innervation of flaps with associated undesirable functional outcomes.