Some Marks and Spencer customers have told the BBC of cases where the chain's contactless payment terminals have taken money from cards other than the ones intended for payment.
Card are supposed to be within about 4cm of the front of the contactless terminal to work.
But some customers say payments have been taken from cards while in purses and wallets at much greater distances.
M&S said its systems had been extensively tested and were robust.
Marks and Spencer recently rolled out the contactless payments system to 644 UK stores.
The system uses something called Near Field Communication to identify a card and take payment.
Rosemary, from Sussex, got a shock when she tried to pay by chip-and-pin at her local store.
She believes her contactless Smile card was much more than 4cm away from the terminal when she visited Marks and Spencer in Chichester in April and tried to pay with her regular Lloyds debit card.
She told BBC Radio 4's Money Box programme: " I put my card into the reader and the assistant was asking whether or not I wanted cash back.
"Before I could answer, the transaction came up as complete and the till issued a receipt so I hadn't put in a Pin at all at that stage. I queried it with an assistant and she looked rather puzzled and looked at the receipt and compared it to my card and realised that the numbers didn't tally."
Rosemary recognised that the four digits on the till receipt belonged to a Smile card she had in her purse, which she was holding in her other hand.
She had not realised until then that this card was able to make contactless payments.
Even when she realised it could, she thought her purse was about a foot away from the terminal when the payment was taken.
Rosemary explained what had happened to a Marks and Spencer manager, who asked her to try to repeat the transaction so he could see it himself.
Again, the Smile contactless card was debited instead of the Lloyds card she had intended to make the payment from.
M&S refunded the two transactions and a third attempt to use the Lloyds card to pay was successful.
Paula, from London, ended up paying for the same items twice.
Like Rosemary, she put her NatWest card into the chip-and-pin terminal but had her HSBC contactless card she was holding in her purse in her other hand debited instead, before she could enter her PIN.
She did not realise that she had a contactless card, and so then paid for the same items by entering the PIN for her regular debit card.
It was only a month later when she looked at her bank statement that she realised that her HSBC card was contactless, and that she had paid twice.
She returned to Marks and Spencer with her statement and it refunded her the payment on her NatWest card.
It's not just Marks and Spencer customers who have experienced this sort of problem.
A Pret a Manger customer also contacted Money Box to report a payment was taken from her contactless-enabled MBNA visa credit card in the outlet when she intended to pay with a different card.
She says the MBNA card was in her purse around 30-40 cm from the contactless card reader.
And again, she had not realised that card had a contactless facility.
Martin Emms is a researcher into new payment formats at Newcastle University's Centre for Cybercrime and Computer Security.
He also found his contactless card was debited when he placed it a few centimetres to the side of the reader from inside his wallet when he intended to pay with a normal debit card.
"If you're placing your card to the side of the reader your intention isn't to pay," he said. "The terminal is working within the specification of Near Field Communication but not within the intent."
Money Box first raised this as an issue with Marks and Spencer back in February. It told the programme the maximum distance its terminal can read a contactless card is 4cm directly above the terminal and the field does not extend beyond the edge of the terminal area. And it said its contactless payment cannot be made if there is another card placed in the reader.
Marks and Spencer said it would like to hear from any customer who had experienced an involuntary payment.
"We are surprised to hear the comments from your listeners, we've had almost entirely positive feedback and take-up has been very strong. We have tested our systems extensively both in labs and in stores and are confident they are robust and fit for purpose.
"A contactless card can be read through material, e.g. a wallet, but the wallet would have to be presented to the terminal. "
The technology used by M&S is provided by Visa Europe.
It said that the customer reports were "extremely unusual", and it was working with M&S and the acquiring bank to investigate the concerns.
Pret a Manger said it had not heard before of customers experiencing this, but it was investigating and it would like to hear from any other customers who have had this happen to them.
Money Box spoke to the banks whose contactless cards had been debited in error.
They said they explained what contactless cards were in accompanying literature when they sent them out.
And they said the effective range of the card was 5cm.
Richard Koch, head of policy at banking industry body UK Cards, admitted that the banks and retailers may have to look at ways of improving the contactless system.
"We're very much in the initial stages of the roll out of contactless. We're monitoring very closely the issues that are presented," he said.