Attempts were made to cover up the mistakes which led to a council paying its former chief executive £98,000, a councillor has said.
A Bristol City Council debate heard the payment given to Anna Klonowski when she left her post was "hush money".
But councillor Clive Stevens said his investigations showed the payout was actually the result of two mistakes.
He supports Mayor Marvin Rees's repeated claim that the council followed legal advice at the time.
Green Party member Mr Stevens spent two months investigating the payment and presented his findings to an audit committee meeting on 30 September, the Local Democracy Reporting Service said.
He said when Ms Klonowski left her role by mutual consent in 2017, a wrong version of her contract was sent to the council's external lawyers.
The lawyers then mistakenly advised she was contractually entitled to the money, said Mr Stevens.
This mistake was compounded when Bristol City Council's human resources committee's policies were not updated to reflect the fact it could have the final say on the payout, despite being granted that power in 2011.
Mr Stevens's report backs up Mr Rees's assertions the council followed the correct procedure but is also critical of unnamed officers or members who tried to cover the authority's tracks, saying they should be "ashamed".
He said: "This whole episode is an example of uninformed decision-making of the highest order.
"I also found evidence of a gross obfuscation. By October 2017, once some legal and HR officers had realised the payment was not contractual, it triggered the start of a cover-up.
"I can't say the mayor approved the payment or even initiated the cover up."
Councillor Richard Eddy, spokesman for the Conservative group on the city council, said: "In my view, it would be a 'red herring' to attribute the scandal to legal and admin errors. At heart, it was the political misjudgement of Mayor Marvin Rees. The latest allegations add nothing to the essential wrongs of this 'golden good-bye'."
Since the mistake, Bristol City Council has changed its procedures so there is more scrutiny of contracts and payouts relating to senior staff.
Mr Rees declined to comment on Mr Stevens's report or Mr Eddy's comments when approached by the BBC.