Definitely the first Android app I decided to fork.
It was an open source OTP authenticator which hadn't been actively developed for 2 years at that point. At first I only did some small fixes and minor visual improvements but by now it's evolved into its own project with a lot of contributores and users on both Google Play and F-Droid.

When I started I had no knowledge of Java or Android development what so ever. So it basically forced me to learn lots of new stuff, especially once issues started to come in. By now I learned so much on this project that I'm thinking about re-writing the whole thing from scratch because I question some of the design choices from the original app I forked...

Github: https://github.com/andOTP/andOTP