Staff at The Holland Hotel in Llanfachraeth, North Wales, took hundreds of pounds in card payments on Saturday, but later realised the card machine had been left on training mode and none of the transactions had been processed.
Managers Liz Mangan and her husband Stephen are now appealing to anyone who used a card on Saturday to "be honest" and come forward.
Mrs Mangan told Daily Post: "On Saturday, we accidentally left our card machine on training mode, which meant that all the payments of all the customers who paid by card that day didn't get processed.
Staff at The Holland Hotel in Llanfachraeth, Wales, took hundreds of pounds in card payments on Saturday, but later realised the card machine had been left on training mode
"We've lost out by over £600, which is obviously a massive blow for any small business.
"It would have been a blow whenever it happened, but to happen on a bank holiday weekend makes it that much worse."
The couple, who have a two-year-old son and previously ran the Jolly Sailor in Prestatyn, took over the Holland 18 months ago.
Following the introduction of a full food menu, its brewery funded a substantial revamp last year.
Liz and Stephen are now appealing to anyone who used a card on Saturday to "be honest" and come forward
The Holland is the only pub in the village and attracts its fair share of tourists due to its location on the A5025, which connects the A55 to northern Anglesey and its many beaches.
Mrs Mangan added: "We've only managed to get £65 back so far, and have been told by the card and insurance companies that they can't do anything to help.
"Staff at the Sail Loft in Amlwch have already given us a donation, which is an amazing gesture and we can't thank them enough. Just a small business helping another out.
"All that we can ask for now is for people to be honest enough so that, once they notice the money hasn't been taken from their accounts, they get in touch with us."