A surgical device was left in a patient's airway after an operation and was only noticed when the person began to gag, an investigation has found.
Details emerged as BBC News NI obtained information on serious healthcare mistakes in NI over the past two years.
The Northern Ireland Ombudsman has also raised concerns that lessons are not being learnt from healthcare mistakes.
The Health and Social Care Board said it was important to have "a systematic process for reviewing and learning".
The device left in a patient's airway was a throat pack, which is used in some surgical procedures to prevent saliva, blood or other surgical debris from tracking down into the pharynx, oesophagus, and the respiratory tract during ear, nose, dental and oral surgical procedures.
Ombudsman Marie Anderson said she was disappointed in the number of complaints about Serious Adverse Incidents (SAI) reaching her office.
An SAI is an investigation carried out by a health trust when unintended or unexpected harm comes to a patient. One of its main aims is to provide learning for the wider health service.
Ms Anderson said she was "constantly seeing SAI cases that lack family involvement and poor standards of investigation".
When an SAI is deemed serious enough, a number of regional alerts - including learning letters and best practice guidance - are sent out to senior healthcare staff across Northern Ireland.
Under Freedom of Information legislation, BBC News NI obtained 18 learning letters and best practice guidance alerts that were sent out between August 2016 and August 2018.
- The patient who was left with a throat pack still obstructing an airway. It was inserted after the patient was anaesthetised. An investigation found that not all medical staff were aware of the presence of the throat pack as there were no visible warnings to alert them.
- A number of patients receiving anti-cancer therapy who were admitted to hospital with diarrhoea and subsequently died. Their deaths were attributed to chemotherapy-related diarrhoea. The seriousness of the patients' diarrhoea was not recognised and inpatient treatment was consequently delayed.
- A patient who had a post-blood transfusion reaction. The mechanism to report such a reaction to the blood bank was not followed. When the patient was admitted a month later for another transfusion, staff were unaware of the previous reaction. The patient died hours later.
Learning from mistakes
About 80% of investigations by the Northern Ireland Ombudsman are healthcare-related.
Since 2014, there have been 21 SAI cases referred to the office, figures obtained by BBC News NI show.
Ms Anderson said many of these cases were not being sufficiently investigated.
"They often fail to address basic things you would expect such as a lack of an independent chair, families not being kept informed and no action plans," she said.
"I've seen SAI investigations that have been conducted by email and clearly that's no way to conduct an investigation, it's important to sit down and talk to people."
'It's been an absolute battle' - One patient's story
Joan Johnson was 81 when she was receiving homecare commissioned by the Northern Health and Social Care Trust.
In February 2015, she had two falls in the space of 48 hours while being raised in a device aimed at helping patients stand during care assistant visits.
A week later, Mrs Johnson died in hospital from a heart condition.
Her daughter, Joan, reported the falls to the Northern Trust, which initiated an SAI investigation.
She said: "Soon after we had our first meeting with the trust they handed us a report as if the issue was resolved, but there was so much left unanswered," she said
"It's been an absolute battle from that minute on."
The investigation took more than two years to complete and there were 18 drafts of the SAI report produced.
Mrs Johnson's family was not satisfied with the outcome of the SAI investigation and the case was referred to the ombudsman.
During the course of the ombudsman's investigation, the northern trust admitted that the SAI process was "deficient", and issued an apology to the family.
Mrs Johnson's daughter believes the SAI process is not fit for purpose.
"I can't grieve because all I can think about is this SAI process," she said.
"It is meant to be about learning for mistakes, but for my family it was almost used by the trust like a smokescreen to make it look like it was doing something."
A spokesperson for the Northern trust said it had met Mrs Johnson's family "to learn how the investigation procedures could be further improved for all patients and their families".
The ombudsman believes poor investigations are having an impact on public confidence.
"There needs to be a change of culture from saying lessons are being learnt to a culture of actually making it happen," said Ms Anderson.
"Only when there is transparency around what happens after an SAI is complete can you start to hold senior officials to account.
"I'm not currently seeing that."
The ombudsman confirmed her office was considering initiating its own investigation into how SAIs are carried out.
The Health and Social Care Board said it took concerns expressed by the ombudsman seriously.
"We would welcome the opportunity to meet Ms Anderson to take on board shared learning and feedback on how the Serious Adverse Incident process can be improved," said a spokesperson.