If Google Sign-In works in local dev but immediately fails with DEVELOPER_ERROR (Code 10) when installed from Play Store Internal Testing, here is why: Most articles and StackOverflow answers tell you to copy the SHA-1 from Play Console → Setup → App Integrity and add it to Google Cloud Console.

Building my React Native app FlashLock, I hit super subtle runtime bug with .env variables. They looked like magic errors at first, but understanding why they happened taught me a lot about mobile JS engines & bundlers! Here’s the breakdown 🧵👇