A woman in her 40s presented with massive upper gastrointestinal bleeding a few months after undergoing a single-anastomosis gastric bypass. CT abdomen and pelvis showed a marginal ulcer with adjacent inflammatory change and suspected colo-enteric fistulation. Endoscopy identified a bleeding fistulous tract; in the context of presumed Roux-en-Y anatomy, this was interpreted as a likely gastro-colic fistula, although diagnostic certainty was limited. Angiography failed to localise active extravasation. Emergency laparotomy ultimately confirmed single-anastomosis anatomy with a gastro-gastric fistula and erosion into the splenic artery. The patient required staged damage-control surgery including resection of the gastric remnant and splenectomy, followed by delayed reconstruction 1 year later. This case highlights diagnostic challenges in altered postbariatric anatomy and the potential for catastrophic vascular complications arising from fistulating marginal ulcer disease.