A health board has been criticised after it emerged it had failed to record some patients' waiting times against Welsh Government targets.
In June it was found that 2,763 patients waiting for treatment or tests at Cwm Taf Morgannwg had been incorrectly left off official figures of more than 62,000 patients.
These include patients waiting for ENT surgery, and kidney and eye treatment.
The health board said changes were "already complete or under way".
Cwm Taf is responsible for Prince Charles Hospital in Merthyr, the Royal Glamorgan near Llantrisant and, since April, the Princess of Wales Hospital in Bridgend.
What was the mistake?
Back in June, 62,914 patients were on waiting lists - being measured against referral to treatment targets.
There were another 51,985 patients whose treatment was not subject to targets.
However, health board managers discovered in this pool of patients there were 2,763 who should have been included within the target.
So, those patients subject to waiting list targets should have been 65,677.
The health board is still looking at whether another 786 people on five different lists need to be included in waiting time targets, but these seem to be patients at low risk.
The picture was already showing worsening waiting times for patients awaiting tests or treatments in Cwm Taf Morgannwg hospitals.
But there are now extra patients recorded as waiting too long.
How was it discovered?
Over the summer, after the concerns were found internally, a team from the Welsh Government's NHS delivery unit was brought in to work with the Cwm Taf to tighten up procedures.
It also coincided with the arrival of a new chief executive.
The team found there had been "insufficient governance" surrounding the administration of waiting lists and this had contributed to a "confused picture" within the health board.
What is the effect on patients?
There would be a concern that those patients wrongly left out might have been given a lower priority and not treated as urgently.
Specialities included ophthalmology, gastroenterology, nephrology (kidney patients), paediatric cardiac, and those patients waiting for diagnosis, including neurophysiology.
It also included 56 patients waiting more than a year for ear, nose and throat treatment and 440 waiting for ophthalmology tests connected with their diabetes.
The review looked at the potential risk to patients and found no evidence of clinical harm.
The delivery unit found there had been "cultural disconnect" between "performance, quality improvement and preserving safety" in some areas of the health board, with concern some patients may have faced the risk of delays in not being subject to a waiting time target.
But the investigation found no indication the health board had been deliberately trying to manipulate the figures with front-line staff acting in "good faith" and in line with processes they believed were appropriate.
Cwm Taf Morgannwg said many of the actions needed to put the situation right were already complete or under way and that measures had been put in place to ensure similar problems could not happen again.