My experience in software development is not related to Tesla but since you asked, I will elaborate a bit: I wrote embedded software that are stored inside the hard disk flash memory. This software control all aspects of the Hard Disk. As more features get added, more subtle issues are introduced, mostly due to unexpected events. Testing are enhanced to cover those events and this is quite an endless loop. But at some phase, the products passed all internal tests and customer tests. Everyone is happy until a newly but very rare event occurs exhibiting a new problem. New version of software , new test, etc... Been in this HDD industry for 20+ years and this cycle never ends.
FSD is a very complicated software IMHO, it's mistakes could be deadly, so I personally do not let it drive my car.
I will create another thread to highlights some weird behavior of TACC and Auto Steer. Don't take me wrong, I love driving Tesla and want to see its software get better and better in assisting the driver's.