Bristol Bears have been deducted five points and given a suspended €10,000 fine by the European Professional Club Rugby for selecting an ineligible player in the European Challenge Cup.
The Bears picked lock Elliott Stooke in their round 1 and 2 fixtures against Perpignan and Zebre in December.
Stooke joined Bristol on 2 December on a short-term deal after leaving Wasps.
EPCR rules state a player must be contracted to a club for a minimum of three months to play.
Despite the five-point deduction, Bristol have still qualified for the knockout stage of the tournament.
Rules also say that clubs must sign an 'additional player undertaking' to ensure the contract remains effective for the three months, to stop them playing anywhere other than the club they have registered with.
An independent disciplinary committee heard that Stooke activated an early release clause in his contract with Bristol to join Montpellier in January after playing in both matches.
The committee accepted that Bristol had "committed a genuine error and had not sought to gain an unfair advantage".
"The committee heard that as all relevant members of Bristol Bears' staff were not aware that Stooke had an early release clause in his contract, the member of staff responsible had made an honest clerical mistake when registering him as an additional player," a statement said.
Bristol have won all three of their Pool A matches in the Challenge Cup and were second in the table having already qualified for the knock-out round of 16 by beating Zebre.
The points deduction means the club have still progressed to the next round but have dropped to fifth in the table with nine points, which may affect whether they get a home fixture in the knockout stage.
The fine is suspended until the end of the 2023-24 season and only needs to be paid if Bristol further breach EPCR rules during that period.
Stooke came on as a replacement for Montpellier in their Champions Cup defeat by Ospreys last weekend.