Almost 1,000 patients were "lost" from a list of follow up appointments causing some "severe harm".
The issue was identified at Southport and Ormskirk Hospital NHS Trust in what managers are attributing to the knock on effect of the covid pandemic.
The patients affected were a group who needed to be followed up and clinically reviewed after finishing treatment or as part of ongoing monitoring of a long-term condition.
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The trust found that 963 patients had been "lost to follow up", and after investigating further classed 22 of those as having experienced some level of harm.
The details were included in a report to the governing body of South Sefton NHS Clinical Commissioning Group (CCG), which stated four of those 22 were found to have suffered "severe" harm, six "moderate" and 12 "low".
The hospital trust says that all the harm occurred before October 2020, when a dedicated team was put in place to track patients on the "non-admitted" follow up list.
A spokesman for Southport and Ormskirk Hospitals said: "Like many NHS trusts, we have reduced capacity due to the impact of Covid-19.
"In line with national guidance our priority is to focus on patients whose clinical need is most urgent.
"This has meant that those patients deemed to be less clinically urgent have waited longer than we would like.
"The figures in the South Sefton CCG paper refer to patients who mostly do not require an inpatient stay and patients who may need follow-up after surgery."
Chief Operating Officer Lesley Neary said: "Since October 2020, we have had a dedicated team tracking this group of patients to ensure we are prioritising them based on their clinical need.
"None of the patients who have been tracked since October have come to any harm.
"We take very seriously and apologise for any time where we fell short of the high standards we set ourselves.
"Each case is thoroughly investigated with our patients kept informed throughout."
The CCG report, by chief nurse Chrissie Cooke, also said the local system is continuing to experience high levels of pressure due to the backlogs caused during lockdown.
Across Sefton as a whole, pressures on the system have seen long waiting lists for patients needing non-emergency treatment.
NHS targets say 90% of patients should receive treatment within 18 weeks of a referral, and a zero tolerance rule is in place for waits of more than 52 weeks.
However CCG figures show that in June this year, 912 patients in Sefton were waiting more than 52 weeks from referral, and only 66% of patients had been treated within 18 weeks.
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