The biggest trap is the "verified but limited" account. Both Alipay and WeChat Pay impose spending caps on unverified or partially verified tourist accounts. You might pass the QR scan test at home (say, paying a Chinese friend back), but at a Shanghai restaurant with a bill above the daily cap, the transaction fails with a generic error message.
To check your limits: In Alipay, go to My -> Settings -> Account Limits. If you see a daily transaction cap under $500 and a per-transaction cap under $50, you are likely on a restricted plan. To raise it, you need to complete full identity verification—which some tourists cannot do without a Chinese bank account. The workaround is to use the Tourist Pass top-up method, which accepts larger amounts from foreign cards but still has a total cap (usually around $2,800 total during your stay).
Another hidden failure is the QR code itself. In Shanghai, many small merchants—fruit stands, street food carts, some metro ticket machines—use dynamic or merchant-specific QR codes that older phone cameras or non-optimized app versions cannot parse. If your verification test only involved a static QR from a friend, you may be unprepared for the real scanner. To reduce this risk, use a modern smartphone with a good camera and the latest version of the app. If possible, test with a merchant QR that you can find online (some travel forums post sample images) to see if your app recognizes it.