Missed medicine deliveries meant some clients of a healthcare firm needed hospital treatment.
IT failings at Burton-upon Trent firm Healthcare at Home meant almost 10,000 medicine deliveries were missed.
Some patients depended on the company for essential medicines, the Care Quality Commission (CQC) said.
The firm, which has has been put in special measures, said the problem had been short-lived and it was disappointed with its rating.
Healthcare at Home provides clinical care in people's homes, including administering essential medicines for a range of chronic conditions including cancer, HIV and haemophilia.
Its services are paid for by the NHS as well as privately funded.
In a report published on Friday, the watchdog said the firm introduced a new IT system in October, but by December had reported 9,985 medicine deliveries had been missed, up from just over 2,000 in October.
It "resulted in avoidable harm to some patients", the CQC said.
An inspection in November and December also found the firm had failed to properly investigate some health and safety incidents and "did not always learn lessons when things went wrong".
It added the company "did not always adequately assess risks to patients, or act on them".
Inspectors similarly criticised the company for lacking an "open culture". It said some patients had said they "feared their medicines deliveries would be cancelled if they complained", while some staff said they feared losing their jobs if they raised concerns or contacted the CQC.
Despite an overall rating of inadequate, inspectors reported some parts of the operation were "good", saying staff effectively followed national protocols and followed up with GPs if they had concerns about patients.
Healthcare at Home said it "deeply" regretted difficulties some patients faced as a result of a short-lived problem with its new computer system in October.
It added that in collaboration with the NHS, the issue was "swiftly dealt with" and within a short time patient service had returned to normal.
CEO Darryn Gibson said the company's top priority had "always been delivering the best possible care and service" and it was disappointed by the rating.
"We remain as committed as ever to providing patients with a high-level quality of service and our new computer system is already delivering an even better service to patients."
The company said it looked forward "to a rapid reinspection".