Doctors at Dunedin Hospital stopped treating an elderly man without looking at a chest x-ray that explained why he was so ill.
Health and Disability Commissioner, Anthony Hill said it was not until after the man died that anyone studied the x-ray.
The 87-year-old man had abdominal surgery in 2012 but was also considered at risk of aspiration pneumonia, which occurs when a patient inhales food or vomit.
He continued to deteriorate however, and doctors decided, along with family, to provide palliative care only, and he died soon after.
The next day staff saw an x-ray revealing the patient had had a very serious but treatable complication of a collapsed lung.
Mr Hill said doctors should have kept an open mind over the diagnosis, and someone should have checked the x-ray.
The DHB's acting medical director of patient services Brendon Rae said all test results were now flagged so they were seen quickly.