Overworked medical staff throughout the UK are endangering patients, according to a report.
Using freedom of information requests to all 172 NHS trusts, the Daily Mirror newspaper found that of 4,000 avoidable errors last year, half resulted in deaths, injuries or severe pain.
Of the 97 trusts that responded, most refused to give details and merely listed fatal mistakes as "unexplained deaths".
The errors included surgeons operating on the wrong person and doctors making the wrong diagnoses or prescribing dangerous doses of drugs.
Dr Peter Carter, general secretary of the Royal College of Nursing, said staff shortages were to blame.
The report referred to a 2001 primary care study report of between five and 80 medical errors per 100,000 consultations, "mainly related to the processes involved in diagnosis and treatment".
These included a wrongly-inserted chest drain puncturing a patient's heart, a patient who had a heart attack and died after a tube was dislodged from his windpipe, and a mother who died of meningitis after giving birth.
The Department of Health said it was working with regulators to monitor improvements.
"Have had experience in my own family when another patient's lens was fitted in the eye of a family member" - Mary, Birmingham
"Secondary care monitoring needs to be more robust. Good communication with honesty and transparency are essentials to assess potential risks. I think in primary and secondary care there should be standard guidelines for assessments for all presenting symptoms which I believe would make clinicians more aware of risk management within the working environment and safeguard the interest of all patients as well as minimise liability and financial expence to the NHS" - V Henry