Once I was hacking an android app to get free coupons. After some investigation I found out they decided to generate the same coupon for n-customers, the coupons were changed every day, the coupon was based on MD5 or SHA1 of generated deterministic value (based on customer ID and date). There were 32k possible coupons per day. App was checking if the account and coupon is valid on their server, but it was always valid as they used the same algorithms in the app and on server... The core of the application was written in C#, launched by Mono on Android.

Java was compiled correctly with ProGuard (code obfuscator), but C# code was compiled in debug mode. C# in debug mode keeps variable/class/method names as they are in source code, leaves comments in the binary, binary file is bigger and slower. You can decompile it to compilable source code, literally.

I managed to get incorrectly protected secret key (they used GPG to encrypt JWT to server lol), later I managed RE of their API, make my own app with identical UI and working coupons. This allowed me to get free coupons every day for next almost 2 years. There were several updates in Google Play since then, but they haven't changed the logic or anything, the app is still in debug mode and anyone can use it. What is interesting, they have also app for iOS which is not in debug mode. Consistency is the key.

After I stopped using their services, I checked their ToS that you have to accept before registering and they prohibit app decompilation, but I never registered and never before read ToS, so it's OK for me ;) I wanted to report the problem but they have literally no contact information, I spent a few hours trying to find their contact details, Twitter is dead since 2014, no contact@ email, no contact form on website... nothing.

Lesson for you: check if your apps are correctly compiled and secrets protected, there are tools for it. In my previous company we had a Jenkins job to test it before release.