Hearts are facing punishment from the Scottish Professional Football League after fielding an ineligible player against Cove Rangers on Wednesday night.
Teenage midfielder Andy Irving played as a second-half substitute but was not properly registered for the Betfred League Cup tie.
Hearts owner Ann Budge has admitted to the mistake and offered apologies, with an SPFL hearing scheduled for Monday to determine sanctions against the Edinburgh club.
The governing body could decide on a fine potentially deduct points from Hearts' total in Betfred League Cup Group C. Ironically, Cove Rangers themselves were fined £2,000 along with Stranraer in 2016 for listing ineligible players in the same competition.
In a statement on the Hearts website, Budge said: "It is with much embarrassment that we have to advise that during last night’s Betfred Cup group stage match away to Cove Rangers, Hearts inadvertently fielded an ineligible player.
"Due to an administrative error on the club’s part at the end of the January transfer window, Andrew Irving entered the field of play in the 65th minute as an unregistered player.
"Andrew was given an extension contract in January, 2018 and his extension paperwork was all properly completed and in order. However, it was not loaded onto the online SFA registration system at the time. His official registration, therefore, ran out on 9th June, 2018. Unfortunately, this was not picked up in advance of last night’s game.
"While stringent processes are already in place to try to prevent something of this nature occurring, clearly mistakes can happen. An additional step in our internal procedures will be implemented immediately to try to ensure this cannot happen again.
"The club would like to apologise to its supporters, to Cove Rangers Football Club and to the footballing authorities for this inadvertent breach of the rules.
"A hearing with an SPFL panel has been scheduled for Monday, July 23rd and will be attended by representatives of the club, where sanctions against Hearts will be determined. Until then, no further comment will be made."