Two patients in Sheffield had the wrong lens put in their eyes in cataract operations within a fortnight of each other.
The mistakes were realised as the surgery took place and were rectified without the patients coming to any harm.
However, investigations are now taking place into how the incidents, which involved different surgeons on April 16 and April 27, happened at the Royal Hallamshire Hospital.
Bosses at Sheffield Teaching Hospitals NHS Foundation Trust say they are ‘very sorry’ and have apologised to the patients.
Both incidents have been classified as ‘Never Events’ – serious incidents that are wholly preventable and have the potential to cause a patient serious harm or death.
Dr David Throssell, trust medical director, said: “We treat more than two million patients every year and our priority is always to provide safe, high-quality care.
“We perform more than 150,000 operations each year and reported three Never Events during 2014-15.
“Regardless of the number, we take every incident seriously and always undertake a full investigation into their causes.
“We are continually examining our processes and checks to try to limit the chances of never events.
“The two incidents involved the wrong strength of lens being used in a cataract operation. However, this was realised during the surgery, rectified and the patients did not come to any harm.
“We apologised to the patients, undertook a full review of the incidents and are implementing changes to reduce the risk of such errors happening again.”
Details of each incident are revealed in a report to the Sheffield Clinical Commissioning Group, which is overseeing the investigations.
A further serious incident also occurred at Sheffield Children’s Hospital in April, when a teenage boy was given a steroid injection in the wrong leg.
John Reid, hospital trust director of nursing, said: “The error was identified by the clinical team, who explained what had happened to the boy and his family.
“No harm was caused to the boy and the correct injection was given immediately.
“We have apologised to the family and thank them for their understanding. We have also looked at our procedures and will be putting additional checks in place.”
Meanwhile, a suspected suicide involving a patient of the Sheffield Health and Social Care NHS Foundation Trust in April is also under investigation after a man died at the city’s Northern General Hospital in Fir Vale in April, following an insulin overdose.
Kevan Taylor, SHSC trust chief executive, said in relation to the suspected suicide: “We have begun an investigation and are due to submit our report to NHS Sheffield CCG by the end of July.”