Hospital staff failed to recognise the serious deterioration of a cancer patient who died the day after he was re-admitted to hospital, a report says.
The public service ombudsman for Wales said he was concerned there had been previous similar incidents at Glan Clwyd Hospital in Denbighshire.
Betsi Cadwaladr University Health Board has been told to apologise to the man's wife, who made a series of complaints.
It said work was being done to "make services safer."
The man, referred to in the report as Mr R, died at the hospital on 29 December, 2008.
His wife complained that when he was admitted for a biopsy on 22 December he was left on a ward for three-and-a half-hours before the paperwork was "shoved under his nose" to sign without proper explanation, said the report.
He was allowed to go home on Christmas Eve, but although he had been having oxygen on the ward he was told there was none available to take home.
He also had a serious infection which had not been diagnosed and there was a possibility of deep vein thrombosis.
When he was re-admitted on 28 December he was not seen by a consultant and the ombudsman found "clinical staff failed to recognise the seriousness of Mr R's condition or his deterioration."
The health board has been ordered to apologise to his wife and pay her £250 for her "time and trouble" in pursuing her complaints.
The heath board's director of nursing Jil Galvani said it accepted the report's findings in full and "standards of care fell below the level patients and their families should expect us to provide."
She added: "Work has already started to make services safer."
She said there was a new consent policy and the timing of consultant ward rounds had been changed to "ensure that new patients are seen by a senior doctor as soon after admission as possible."
"Over the coming months, the health board will be checking to make sure that these systems are making a positive difference," she added.