Straying slightly from your regular "accepted" and "declined", some Eftpos machines have been delivering an odd message.
Usually, the terminals list the name of the business or payments provider on the home screen.
IT News said the machines had displayed their new messages for several weeks, but New Zealand retailers were not concerned.
"When I saw the screen I laughed and drew the attention of the cashier to it," one customer told IT News, who noticed the message at a cafe in Rotorua.
"She looked at it and said "oh yeah" in a casual manner that implied to me she didn't think it unusual."
Another shopper in Takapuna saw the new message.
"The cashier asked why I was laughing and said "Oh, is it still saying that weird thing?" She had no idea what it was. I told her it was just a geek joke and that it was probably their IT guy playing a prank," the customer said.
Direct Payments Solutions, the technology supplier, told IT News a test message had been pushed to some terminals, but could not say when they went through.
Payment Express is used by Visa, MasterCard, American Express, and Diners.
The phrase Derp is usually associated with stupidity and is believed to have been coined by South Park creators Trey Parker and Matt Stone, according to the website Knowyourmeme, and is normally accompanied by an amusing picture of a celebrity or other simple cartoon figure.
One buyer tweeted a picture of the screen, saying " This just happened to me. Herp-Derp Eftpos. Respect, Mr Eftpos installation man."