Pneumoperitoneum on a CT of a critically unwell patient raises suspicion of hollow viscous perforation. However sometimes this air has an alternative source – anaerobic infection. This report presents the case of an elderly gentleman with acute necrotising pancreatitis infected with Clostridium perfringens masquerading as duodenal perforation. The correct pathology was only identified when exploratory surgery revealed an intact duodenum and peripancreatic fluid collection with microscopy confirming the anaerobic organism. This case serves as a reminder to consider all possible causes of air in the abdominal cavity.