when u break some functionality on prod as you and QA both missed a test case

How dare users test a case without informing you both!

Test caeses are only catching bugs you thought about. To catch the bugs you didn't anticipate, you have to use static analysis and real humans actually using the software for what it is intended to be useful for.

Dev: builds it, tests it under happy paths and some unhappy paths.

QA: throws all kinds of strange data at the solution.

User: finds the one fucking piece of data that breaks it beyond a joke within minutes of going live.

Dev: spends a week debugging problem.

Qa: retests under old use cases + newly identified one

User: finds another bug......

Long story short, it's a fucking cycle of insanity until the big reports die off.