A 46-year-old man with metastatic lung adenocarcinoma was treated with pembrolizumab. FDG PET/CT was performed after 3 cycles of treatment and revealed a focal region of pancreatic tail enlargement with an SUVmax value of 7. Following treatment with corticosteroids and discontinuation of pembrolizumab, radiological resolution was observed, and a diagnosis of focal immunotherapy-induced pancreatitis was made. A unique spectrum of FDG-avid adverse events can develop in patients treated with immune-checkpoint inhibitors that may mimic metastatic disease. Knowledge of the radiologic features of these potential imaging pitfalls is crucial among those interpreting FDG PET/CT to allow prompt and decisive treatment.