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
Gavin Thorman, 37, from Caernarfon was jailed for 12 years last year for his part in conspiring to supply Class A and B controlled drugs.
Jeremy Corbyn has hailed the High Court's decision to block a 'waste of time' legal challenge to his place on the Labour leadership ballot.
Scott Horwood from Burry Port has made the working replica, which rolls card instead of steel. Obviously.