The Day I Chased a “Ghost” Bug in Real-Time Notifications

The Problem That Wasn’t There… or Was It? A few months ago, I was working on a mobile app with real-time notifications. The feature seemed straightforward: whenever a new order was placed in the e-commerce app, the user should receive a push notification instantly. Everything worked perfectly during my development. On my devices, notifications popped up within milliseconds. But when the beta testers started using the app, something strange happened: Some users received duplicate notifications. Others got notifications late, sometimes even minutes after the event. A few users never received them at all. It felt like chasing a ghost. How could the system behave so inconsistently? Digging Into the Mystery The app used Firebase Cloud Messaging (FCM) for push notifications. At first, I suspected the backend, the mobile framework, or even the users’ devices. I went step by step: Checked the backend logs—all notifications were sent correctly. Tested on multiple devices—duplicates appeared onl