St Helier Hospital conducted a serious incident investigation after a swab was left inside a person’s body.
The "never event", so serious it should never have happened, occurred in December 2012.
Three days after the initial procedure the patient was returned to theatre for exploratory surgery and a swab was identified and removed from their abdomen.
The Epsom and St Helier trust immediately launched a review into the incident led by an executive director and a clinical director.
The investigation identified human error following a failure to follow trust procedure. Staff had failed to complete the WHO checklist, a surgical safety list, and to fully complete the local checklist which includes a requirement to document swab counts.
Immediate actions were agreed including no patient to leave theatre before all documentation was completed and the staff involved in the incident were removed from theatre duties until further notice.
The chief executive Matthew Hopkins has apologised for the failings in the patient’s care and an action plan has been agreed which details 22 actions to be taken forward by senior managers and clinicians within the trust.
Never events hit the headlines earlier this year after NHS England admitted the figures were too high and it had introduced new measure to ensure patient safety.
A spokesperson for the Epsom and St Helier trust said: "Our busy hospitals have a record of providing our patients with a high level of care, and meeting the vast majority of the standards that the Government expect of us.
"As such, any incident that may impact on the health, safety and wellbeing of our patients – however rare – is taken very seriously indeed.
"In the past three years, we have treated approximately 2.4 million patients and have reported five never events. Whilst we recognise that this is a very small proportion of the total patients treated, we will not hesitate to act upon and learn from these incidents.
"Any untoward incident is subject to a thorough internal investigation at the trust, and as a priority, actions and measures are put in place to help prevent a similar event occurring."
Epsom and St Helier Hospitals recorded five "never events" in the last four years. In comparison Croydon Health Services NHS Trust recorded eight incidents, Kingston Hospital NHS Trust recorded four incidents and St George's NHS Trust recorded eight incidents.
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