A person having the wrong tooth pulled out was among three "never events" at the Royal Cornwall Hospitals Trust (RCHT) in just one month.
The wrong tooth was taken out during surgery under local anaesthetic, however the patient later returned and had the right one removed.
Never events are "serious, largely preventable patient safety incidents that should not occur if the available preventative measures have been implemented".
A "rigorous investigation" is underway at the trust as a matter of routine when "never events" are reported, with a trust spokesman saying: "Patient safety is our top priority and we are clear that any never event is unacceptable and are always subjected to a rigorous investigation process to ensure RCHT and, where appropriate, the wider NHS learns from any incident."
The patient returned to clinic where the correct tooth was removed.
The second never event involved an incorrect implant and the third was a case of "wrong site surgery", which is defined as a surgical intervention performed on the wrong site (for example wrong knee, wrong eye, wrong patient, wrong limb, or wrong organ).
The trust was told it had to improve following an inspection on 21-22 and 25 January this year.
There also 75 "serious incidents" involving patients, visitors or staff, over the past year.