This article originally appeared on ABACUS It’s annoying when any app crashes on your phone, but most of the time it’s just a minor inconvenience. What happens when the app is an integral part of your life, though? Some Chinese commuters learned the hard way Monday morning. In the southwestern metropolis of Chengdu, frustrated subway riders took to Weibo to complain that a transport payment app called Tianfu Tong wasn’t working. Given the size of the city and the popularity of mobile payments in China, a disruption such as this can lead to large-scale delays. Chengdu is a massive city with 16 million people, about four times the population of Los Angeles. Like many mobile payment services in China, Tianfu Tong normally generates a unique QR code for each transaction. Train passengers, for example, can use it to scan in and out of the station. How the QR code conquered China The problem this time was that the app failed to load a QR code. Unable to pay digitally, commuters were forced to resort to using an old-fashioned, physical fare card. Lines formed in front of ticket machines and counters as cards were being refilled and new tickets purchased. Some commuters didn’t even have that option. Some Weibo users said they were stuck without cash -- an understandable predicament in a country where nearly 70% of internet users use mobile payments . The unexpected delay meant some passengers showed up late to work. In an official apology posted on its Weibo account , Tianfu Tong pledged to publish a letter that affected riders can download and present to their employers as proof. That didn’t seem to be enough to pacify angry riders, who pointed out that the Chengdu subway doesn’t accept Apple Pay and several other payment apps that use NFC -- a more secure alternative to the QR code. Even though Apple Pay has struggled to catch on in most of China , it’s available to commuters in Beijing and Shanghai . Major Chinese phone makers like Xiaomi also support NFC payments for public transport. QR codes have enjoyed wide success in China, being adopted by the country’s two most popular payment apps, WeChat Pay and Alipay, owned by Tencent and Alibaba respectively. The technology is relatively cheap and easy to adopt, but its safety has come under scrutiny . Others have also pointed out that QR code scans are slower than NFC, making the former less than ideal for rush hour travels. (Abacus is a unit of the South China Morning Post, which is owned by Alibaba.) While Tianfu Tong is busy sorting out this mess, some commuters in Chengdu say they will just stick with the tried and tested technology of paper metro cards. “Am I old? I always bring my transit card when I go out,” said one Weibo user . For more insights into China tech, sign up for our tech newsletters , subscribe to our Inside China Tech podcast , and download the comprehensive 2019 China Internet Report . Also roam China Tech City , an award-winning interactive digital map at our sister site Abacus .