Professor Andrew Wilcock said notes were not taken properly
A patient being treated for a heart condition at a Hampshire hospital also suffered from internal bleeding which was not diagnosed, an inquest heard.
The jury in the inquest into 10 deaths at the Gosport War Memorial Hospital more than 10 years ago heard Geoffrey Packman's blood tests were not checked.
Professor Andrew Wilcock said Mr Packman, 66, was also given "excessive amounts" of diamorphine for his needs.
He died nine days after arriving to the hospital's Dryad Ward.
Professor Wilcock, an expert in palliative care, told the inquest at Portsmouth Coroner's Court that Mr Packman had been admitted for rehabilitation and help with his mobility and he was not terminally ill.
He said there had been a lack of clear note taking when dealing with patients, which also failed to justify some of the doses of diamorphine being prescribed.
Mr Packman was suffering from "serious internal bleeding" but this was never properly diagnosed because his blood test results were simply not read, Professor Wilcock said.
Geoffrey Packman died nine days after arriving at the memorial hospital
He said it was possibly a peptic ulcer - a condition which could have been treated.
But the jury heard it was serious and may have caused Mr Packman's death.
Professor Wilcock also spoke of the case of Arthur Cunningham, who died after being admitted seriously ill with bed sores.
He acknowledged that Mr Cunningham "was nearing the end of his life", but questioned the dose of diamorphine administered to him.
He described it as "extreme and a large increase which was difficult to justify".
Some families also believe sedatives were over-prescribed at the hospital.
Hampshire Police carried out a series of investigations into the treatment of patients at the hospital in the late 1990s, but no prosecutions have been brought.
The inquests are being held into the deaths of Arthur Cunningham, 79, Elsie Devine, 88, Sheila Gregory, 91, Ruby Lake, 84, Elsie Lavender, 83, Geoffrey Packman, 66, Leslie Pittock, 83, Helena Service, 99, Enid Spurgin, 92, and Robert Wilson, 74.
The hearing continues.