The Senators traded away Evgeni Dadonov to the Vegas Golden Knights in the summer of 2021 to the Vegas Golden Knights. At this time Dadonov had a modified no trade clause.
With modified no trade clauses the player can pick and choose where he does not want to be traded to. Vegas was under the impression that Dadonov had not submitted his list for the 2021-22 season and tried to trade him to the Anaheim Ducks.
However, Dadonov had submitted his list to the Senators who proceeded to not share that with the Golden Knights, the NHL stepped in and vetoed the trade to the Ducks and have now punished the Senators for it.