It is estimated that 5,000 people in Wales who are paying a spare room subsidy could - and should - have been exempt.
Under the rule, anyone who has been in their home since before 1996 and has continually claimed housing benefit does not have to pay the subsidy.
A spokesperson for the Department of Work and Pensions say they are looking at the issue carefully and will take any necessary action.
More top news
A dry night with clear skies across much of Wales. Temperatures will fall widely below zero, so if you're out and about, wrap up!
An orphaned seal pup has been returned to the wild after a spell in RSPCA rehabilitation.
Dutch-owned Abellio Rail Cymru (ARC) has ended its tender for for the next Wales and Borders rail franchise.