The government's Test and Trace app has given a women the results of a Covid test she said she had not taken.
Elaine Watson, from Middlesbrough, was told by the app to self-isolate for seven days and then, on day six, that her test had come back negative.
"I'm thinking, what do you mean a test? I haven't had a test," she said.
The Department of Health and Social Care (DHSC) said the app was constantly being improved in response to feedback but most users had "no issues" with it.
Ms Watson, a member of BBC staff, said she was not contacted by Test and Trace and had not taken a coronavirus test because she had no symptoms.
"Was that message meant for somebody else? Did I really need to isolate?" she asked.
Other people have said they were sent results for tests they had not taken and some have taken a test but received no result.
Alex Weedon, from Colchester, was emailed by Test and Trace to say he had coronavirus, despite not having taken a test.
Trying to track down who the intended recipient had been was "baffling", he said.
Test and Trace has failed to contact about 19% of people whose cases have been passed to it after testing positive.
Between 28 May, when it launched, and 18 November nearly 17% (160,569) were not contacted. For 20,091 of these, "communication details not provided" was given as the reason.
The DHSC said everyone who tested positive was informed, but it was not clear on how a person's contact details could become "not provided" between them being informed and their case being passed to contact tracers.
British Medical Association North East chairman Dr George Rae asked why contact tracers could not call the person on the number used to text them their results.
"That doesn't make any sense at all, does it?" he said.
One contact tracer from Teesside, who wished to remain anonymous, said he was not able to reach about 60% of the people he tried to contact.
"It is probably even higher than 60% that aren't reached, but I try to be at least a little optimistic," he said.
Another contact tracer said there was "no clear guideline" on how many attempts should be made before giving up.
If a person could not be reached there was no way of knowing whether the tracer had the correct number, he said.