More than 13,000 patients were wrongly taken off a hospital waiting list due to persistent IT problems, councillors were told.
The NHS trust that runs hospitals in Grimsby, Scunthorpe and Goole said coding errors had led to “clock stops” being applied to 13,475 patients over 10 months.
The chief executive of North Lincolnshire and Goole Foundation Trust, Dr Peter Reading, told East Riding councillors on Tuesday, NHS staff had “still not bottomed” out IT issues.
Patients has been incorrectly recorded as having had treatment and taken off a particular list - but not removed from the waiting list completely.
Dr Reading said, following checks, managers were confident patients had not come to harm. He described it as an “administrative issue” involving a “tiny risk” to patients.
However, he could not guarantee further IT problems would not emerge, given recent history.
He said NHS managers are planning to restructure the IT system over the next two years, adding: "In the meantime we are putting in a lot of measures that things that might have gone wrong, can no longer.”
Councillors heard that the trust, which is in special measures for finance and quality, had a £58m deficit, up from £51m, following a downturn at the end of the 2018/2019 year.
It was visited by 72 inspectors from the Care Quality Commission three weeks ago, followed by an unannounced inspection last week.
Dr Reading, who joined the trust in August 2017, told councillors inspectors had noticed their “substantial” progress in nursing.
Medical vacancy rates have improved from 22 per cent to 14 per cent, and staff turnover is down from 12 per cent to eight per cent.
He said: “Two years ago we had quite destructive turnover rates – our staff survey was the worst in the country. Too many people said this is a place where bullying is prevalent, (both) managerial and within teams.
“Quite a number of wards had a toxic culture, cyberbullying and all sorts of behaviour. It is not unique to us – but I think we were worst than most.”
The trust now runs a “pride and respect” programme and people are encouraged to “speak up”.
It has also gone in 18 months from having the worst waiting list in the country, to around 24,500 patients, “probably 20 to 30” from the bottom of a list of 140 trusts.
However, only around two-thirds of patients are treated within 62 days following an urgent GP referral for suspected cancer. The Hull trust’s record is worse.
Dr Reading said their ambition was to exit special measures in 2020 – but the following two years would be crucial to ensuring changes in culture were bedded in.
He said: “What we are nervous of is making changes that look good in the short term, but don’t have proper foundations.”