Accidents, errors and mishaps in hospital affect as many as one in 10 in-patients, claim researchers.
Errors in treatment are common
The report in the journal Quality and Safety in Health Care said up to half of these were preventable.
Checks on 1,000 cases in just one hospital found examples of fatal surgical errors, infections and drug complications.
Researchers from the University of York experts say more should be spent on monitoring "adverse incidents".
Skin burned with diathermy tool during operation
Delay in cancer diagnosis
Bleeding from penis after catheter removed without deflating balloon
Spleen torn during operation - patient needed six litres of blood to survive
Patient addicted to painkillers after high dose continued after discharge
The government has encouraged trusts in recent years to spend more effort looking at complications and mistakes involving their patients.
Managers are supposed to report even "near misses" in which patients suffered no harm, so that lessons can be learned.
However, other studies have suggested that the reporting rate is poor. The University of York study focused on a single major acute hospital in England, and pored over the notes of 1,006 people admitted into it.
While 87 people had definitely suffered at least one "adverse event", the researchers said it was likely that even more had suffered harm.
Alongside more than 40 infections, there were 27 complications during or following operations, 19 drug complications, and 12 cases of bedsores. Between 30% and 55% of these could have been prevented by clinical staff or managers.
Examples of preventable incidents included a mistake in an operation which led to the death of the patient, another which caused lifelong damage, and a case in which a patient became addicted to opioid drugs after being given a high dose during and after a hospital stay.
Professor Trevor Sheldon, who led the research, said that "finger-pointing" was not the answer - although the scale of the problem meant that more resources should be spent tackling it.
"The rates we found do not show that the NHS is faring worse - this is an international issue, and other countries have similar or worse rates.
"The question we have to ask is whether the NHS is currently doing enough to help people find the time to reflect on these cases and learn lessons from them.
"Our research does confirm though that hospitals are not completely safe places, and that people should try to steer clear of them unless absolutely necessary."
A spokesman for the Department of Health said that the creation of the National Patient Safety Agency (NPSA) in 2001 was designed specifically to improve the NHS response to adverse incidents.
"We have long recognised patient safety as a top priority, and it is important to remember that serious failures are uncommon in relation to the volume of care provided by the NHS.
"As the study suggests, many adverse events could be avoided if lessons were properly learned and fed back into practice."
A spokesman for the NPSA welcomed the study, and said it was working with the NHS to improve safety.
"Around 13 million people are admitted to acute hospitals each year in England and Wales. Most people are cared for safely, however regrettably sometimes things can and do go wrong."