The Bug That Only Happened in Production (and Only on Tuesdays)

A few months ago, I ran into one of those bugs that makes you question your sanity. Everything worked locally. Everything worked in staging. But in production… some users couldn’t log in. Not all users. Not consistently. Just sometimes. 🧩 The Symptom We started getting vague reports: “Login fails randomly” “Works after refresh” “Only happens sometimes” No errors in logs. No obvious crash. Just silent failure. Which is always worse. 🔍 First Clue After digging into logs more carefully, I noticed something odd: Failed requests had empty request bodies But only on POST /login That didn’t make sense. The frontend was definitely sending data. 🧪 Reproducing the Issue Tried everything: Different browsers ✅ Incognito mode ✅ Slow network simulation ✅ Nothing. Then one teammate casually said: “I saw it once on mobile.” That changed everything. 📱 The Breakthrough I tested on mobile + weak network. Boom. Reproduced. Now I could finally see what was happening: Request was sent But sometimes the body