A testicle removed instead of a cyst, diabetic patients not being given insulin and scalpels being left inside patients' bodies are some of the shocking cases that more than 1,000 patients have suffered in hospitals, an investigation has found.
The "never events" - mistakes so serious they should never happen - include a woman having her fallopian tubes removed instead of her appendix and a patient having a biopsy taken from their liver instead of their pancreas.
Hundreds of patients have suffered from the serious errors in England's hospitals over the past four years, The Press Association discovered.
More than 1,100 patients have suffered 'never again' incidents in England's hospitals
More than 400 people have suffered due to "wrong site surgery", while more than 420 have also had "foreign objects" left inside them after operations - including gauzes, swabs, drill guides, scalpel blades and needles.
In one case, a man had a testicle removed instead of just the cyst on it, while a woman had a kidney removed instead of an ovary.
Other patients have suffered when feeding tubes which are meant to be fed into their stomach have been put into their lung instead. This can prove fatal.
Others have been given the wrong type of implant or joint replacement, some patients have been mixed up with others, and some patients have been given the wrong type of blood during a transfusion.
Some patients have also been given far too high doses of drugs, including oral methotrexate, which is used for the treatment of severe arthritis, psoriasis and leukaemia.
In other cases, prisoners have escaped and some patients have fallen from poorly secured windows.
Katherine Murphy, chief executive of the Patients Association, said: "It is a disgrace that such supposed 'never' incidents are still so prevalent. With all the systems and procedures that are in place within the NHS, how are such basic, avoidable mistakes still happening?
"There is clearly a lack of learning in the NHS.
"These 1,100 patients have been very badly let down by utter carelessness. It is especially unforgivable to operate on the wrong organ, and many such mistakes can never be rectified."
The analysis of data published by NHS England shows there were 254 never events from April 2015 to the end of December 2015.
From April 2014 to March 2015, there were 306 never events, and from April 2013 to March 2014, there were 338.
In the previous year - from April 2012 to March 2013 - there were 290.
In 2014/15 Colchester Hospital University NHS Foundation Trust was the worst hospital trust, with nine never events recorded.
The data for April to December 2015 is still regarded as provisional.
Royal College of Surgeons president Clare Marx said: "This data shows an unacceptable level of preventable mistakes are still happening in the NHS.
"While these cases are very rare, never should mean never. Learning from mistakes and using best practice and guidance to avoid such errors should be the priority of every medical and surgical team across the country.
"The NHS must continue to learn from these errors so we can become the safest healthcare system in the world."
Never events are incidents that should never occur as guidance is in place to prevent them.
An NHS England spokeswoman said: "One never event is too many and we mustn't underestimate the effect on the patients concerned. However there are 4.6 million hospital admissions that lead to surgical care each year and, despite stringent measures put in place, on rare occasions these incidents do occur.
"To better understand the reasons why, in 2013 we commissioned a taskforce to investigate, leading to a new set of national standards being published last year specifically to support doctors, nurses and hospitals to prevent these mistakes.
"Any organisation that reports a serious incident is also expected to conduct its own investigation so it can learn and take action to prevent similar incidents from being repeated."
The never events list for 2015/16 - which hospitals use to determine whether there has been a never event in their trust - has been cut to 14 events.
Officials have removed several never events, including the death of a mother from a haemorrhage after a planned Caesarean, saying such an incident is not wholly preventable.
The escape of a transferred prisoner will now also be regarded as a "serious incident" rather than a never event, while a failure to monitor and respond to oxygen saturation levels is no longer regarded as a never event due to the belief that protocols are not strong enough to prevent it happening.