The BBC's botched Newsnight programme failed to complete "basic journalistic checks", an official report has concluded.
And there was confusion about who had the ultimate responsibility for "final editorial sign-off" on the story which mistakenly implicated Lord McAlpine in a child sex abuse scandal.
A report by the BBC's Scotland director Ken MacQuarrie into the Newsnight blunders found the programme's editorial management structure had been "seriously weakened" as a result of the editor having to step aside over the Jimmy Savile scandal, and the departure of the deputy editor.
The mistakes led to the departure of director-general George Entwistle on Saturday night.
The problems in the programme stemmed from two different sign-off processes being put in place as a result of continuing inquiries into Savile. Mr MacQuarrie's report said there was a "separation between 'business as usual' stories and 'Savile-related' stories", with a separate chain of command for anything to do with Savile.
He said: "It was not clear whether this story was regarded as Savile-related or not, or when that decision was made and communicated: a clear decision on this does not appear to have been taken until lunchtime on Friday 2 November. As a consequence there was ambiguity around who was taking the ultimate editorial responsibility for the Newsnight report, particularly in the days leading up to the day of transmission."
Mr MacQuarrie went on: "There was a different understanding by the key parties about where the responsibility lay for the final editorial sign-off for the story on the day." And he said there were shortcomings in the quality of the journalism.
"During the editorial decision-making process, some of the basic journalistic checks were not completed," he said. "Specifically, identification was not confirmed by photograph with the first victim. The second victim could not be traced in order to provide up-to-date corroboration."
Although legal advice was sought over the report, no right of reply was offered to the unnamed individual at the centre of the allegation. The programme featured an interview with Steve Messham, an abuse victim who said a senior political figure of the time had abused him. He later said he had wrongly identified his abuser and apologised.
The BBC called the failings "unacceptable". Earlier it brought in a single chain of command for news output once again. The BBC Trust said Mr MacQuarrie's findings of "serious failures" were "very concerning".