You wrote your first REST endpoint. You ran your first SELECT *. Things worked. Life was good.
Then a colleague casually mentioned "idempotency key," another asked why your /users?page=10000 request takes 12 seconds, and your tech lead frowned at your migration script. Suddenly, "it works on my machine" is not enough anymore.
I have been through that frowning-tech-lead phase. So has every senior engineer you know. The mistakes are surprisingly predictable — and almost all of them are easy to avoid if someone points them out before you ship.
This post is that someone.
Below are 10 mistakes I see beginners make again and again with APIs and databases. For each one, you get the blunder, why it bites, and the actual fix.
