Electronic Heart Record system for Nigerian institutions as a project lead. These things are hell of complications.

Worked on a Fleet Management System back in 2009, in Bangladesh. Both the tracking devices and map was created in-house. I was responsible for the map design (Mapnik), GPS Server (Netty), and the web application (full-stack), I would call it complex because we had to make some really interesting (at times impractical) reports, enriched the system with some cool abilities (stopping the car with an SMS, locking the door etc), and an interesting GeoFence system. Also, it was my first “real” job.

Well at least that system I helped build survived till 2013 and served ~10k vehicles before being completely rewritten, and it’s going great now I heard (as I tried selling Elixir to the CEO and got him learning it at least). Both the in-house members (device and map) were shelved

Mine is an e-commerce app with an elaborate referral scheme, however I think the most pita type of apps (or part of an app) has been those where you are working with external APIs or services! You can’t just simply test things in the console as the result may vary depending on the service you are working with although maybe not quite direct complexity but definitely makes it feel that way