A young woman with autism living in a learning disability unit was left blinded in one eye after an injury was left untreated for two months.
The 24-year-old was in a unit run by Swansea Bay health board when she injured her eye in June 2018.
But an ombudsman's report found she was not treated until September and lost the sight in her right eye.
The health board apologised and said it was "determined" nothing like this would happen again.
The report said the woman, who was not identified, was living at the residential unit when she hurt her eye.
The woman, who has autism, learning difficulties and mental health difficulties, was known to injure herself, including hitting herself on the head and face, causing bruising.
But the report states that while staff "noted concerns" which required monitoring, there was no evidence checks were made, or the concerns were escalated for treatment.
After a doctor raised concerns in September she was taken to hospital where she was diagnosed with total retinal detachment and traumatic cataract of the right eye, and an consultant said she no longer had sight in her eye.
Her mother believed that her daughter's eye injury had been left untreated for six weeks after she hurt it, and contacted the Public Services Ombudsman for Wales, who has legal powers to investigate complaints.
The report said she thought her daughter "was young to lose her sight in the way she did" and "she found this heart-breaking and difficult to accept".
The ombudsman found good care had been provided at the unit for the woman's specialised learning disability needs but there were "serious shortcomings" in the care relating to her eye treatment.
The report said it was impossible to say if an earlier referral would have saved her sight but she had been denied the opportunity of a timely referral and clinical review.
Nick Bennett, Public Services Ombudsman for Wales, said: "Individuals in institutional care settings are amongst the most vulnerable in society and public bodies need to be extra vigilant to ensure their needs are met.
"This is an extremely serious case where a young woman has been left with a permanent life-changing injury that may have been avoided."
The health board said all the changes recommended by the Ombudsman were being implemented and it was determined to make sure it did not happen again.
Christine Williams, Swansea Bay UHB's Interim Director of Nursing and Patient Experience, said: "Unfortunately the care provided did not meet the standard the health board expects its patients to receive.
"We do not take these recommendations lightly. Neither do we underestimate the impact the failing in care has had on the patient and her family.
"We can assure them we are determined to do everything we possibly can to ensure this does not happen again."