Staff at Kingston Hospital operated on the wrong body part, wrongly-inserted a feeding tube and left objects inside patients twice, during 2011 and 2012.
The four “never events” are patient safety incidents which should never happen if preventative measures are followed correctly.
A hospital spokeswoman said it had the lowest number of never events in south-west London.
She said: “All the never events are thoroughly investigated by a panel chaired by a non-executive director and are discussed in full, including agreeing the action plan, during the private part of the trust board meetings.
“Any member of staff involved in a never event has this recorded on their personal record and lessons learnt are shared with staff to prevent a similar incident happening again.”
The number represents 0.0001 percent of patients who receive elective or emergency treatment each year, the hospital said.
Patients affected by never events are involved in investigations, it added.
Nora Pearce, Unison steward at Kingston Hospital, said: “I think what it does do is highlight to everybody that it is possible to make mistakes, and it does make you more vigilant. People would be shocked that it did happen.
“The world and his mother checks that this is the right arm, the right leg – it is checked a million times.”
Julie Reay, co-founder of Health Alert Kingston, said hospital workers have expressed worries that a fewer nurses could lead to more serious incidents.
She said: “They are really feeling the pressure. It is just not sustainable and it is really important that nurses feel safe and secure.
“I think the overall picture is more scary.”
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