Legal advice was sought by Swansea Council after it mistakenly gave planning permission for a house of multiple occupation (HMO) in the city's Eastside.
The area, which has become popular with students, is subject to a new city-wide policy on HMOs to help control their spread.
It means that no more than 10% of all residential properties within a 50-metre radius can be HMOs, except a designated area of Uplands where the threshold is 25%.
It has emerged that a HMO wasn't picked up by the planning department when officers assessed a zone around a house in Crymlyn Street, Port Tennant .
If it had, the 10% threshold would have been exceeded and HMO planning permission for 35 Crymlyn Street would have been refused.
Cabinet member for delivery, councillor David Hopkins, said: "There was a mistake made. It was a one-off. But I think the policy is right."
Since coming into effect earlier this year, HMO applications have been turned down and dismissed on appeal based on the new policy, which is part of Swansea's new local development plan (LDP).
The first was in May when a Welsh Government planning inspector dismissed an appeal for a six-bedroom HMO on St Helen's Avenue, Brynmill .
St Thomas councillor Clive Lloyd said he and his ward colleague, councillor Joe Hale, had queried the 35 Crymlyn Street HMO application.
"To say that we were hugely disappointed that such an error occurred so soon after the LDP had been passed is an understatement, especially after we campaigned so hard for HMO restrictions to be put in place," said Cllr Lloyd.
"We challenged the council to overturn the decision, and have sought assurances that this is a complete one-off and will not happen again."
Cllr Hale said: "It's really unfortunate. Mistakes do happen, and I hope we learn from our mistakes."
The council, which said the legal advice it sought in relation to the Crymlyn Street decision was confidential, will publish supplementary planning guidance shortly to help clarify the policy.
Cllr Lloyd said additional licensing requirements for HMOs in St Thomas and Port Tennant would be discussed by cabinet in due course which, if adopted, would provide additional safeguards.
