When Les Howard could not find change for parking during a trip to the coast, a quick response (QR) code on the meter seemed to be the answer.
But the next day, the 61-year-old realised he had become the victim of an increasingly common scam as criminals started taking money from his bank account in £60 chunks.
Reports of QR code-related scams have gone up by 700% in the past four years, figures by Report Fraud have revealed.
Experts say criminals are finding different ways to exploit the codes as they become a common part of everyday life.
This could be disguising suspicious links in emails, gaining access to victims' messaging apps or convincing people who are out and about to download fraudulent apps to steal their money and data.
That is what happened to Howard, from Widnes, who was visiting Caernarfon in North Wales with his wife.
"We went to the car park where we normally go," Howard told the BBC Scam Secrets podcast.
"I normally have change in the car to pay for the parking, but this time I didn't.
"So there was a QR code to download and get an app and pay via an app."
Other people in the car park were doing the same thing but Howard recalled how he became suspicious.
"I thought 'I don't like the idea of this because I'm going to have to put my card details into an app that I've only just downloaded, which wasn't of my choosing'."
So he stopped, deleted the app from his phone and parked somewhere else.
But by then he said he received a notification on his phone that £1 had been taken from his account.
The next day, another notification came through, but this time for £60, to the same recipient so he called his bank.
"I told them I'd downloaded it, I tried to stop, but I'd obviously gone too far," he said.
"She said 'it's a good job you rang up, because they've set up a monthly £60 subscription'."
The council responsible for car parks in the area, Cyngor Gwynedd, said it did not use QR codes for parking payments, and had removed two fake QR code stickers from its car parks in the past year.
Criminals have used similar tactics in car parks and railway stations across the country.
Data from City of London Police released under the Freedom of Information Act suggests the problem of QR code-related scams - known as "quishing" - is growing.
The NCSC said QR codes were also increasingly being used in phishing emails to disguise links to malicious sites.
Microsoft said this was the fastest-growing scam technique aimed at people's emails, with 18.7m cases recorded in March of this year.
This is because QR codes allow scam links to "slip through" on email, while a URL embedded in a message might be filtered out as spam, said Professor Filipo Sharevski of DePaul University in Chicago, who has studied the malicious use of QR codes.
The black and white squares also make it easier for criminals to hide the true website they are taking you to, he said, because even if your phone lets you preview the link before clicking, it is often shortened or disguised.
Another reason QR codes are so valuable to criminals is that people are often naturally trusting of them, Prof Sharevski said.
His team has carried out experiments placing QR codes around his university campus and in workplaces, to study people's willingness to scan them.
"We don't question our boarding pass on our phone, we don't question our concert ticket," he said.
"We learn slowly through scam experiences. Our phone rings… we abandon that.
"We get an email, and we abandon that…but these QR codes, we have no reason to suspect them."
