[UweSchmidt]

Goes without saying that you deploy to an identical test environment that duplicates the entire stack, with realistic test data. Automated testing protects from regressions, manual testing for the rest.

crickets

One (QA) can always dream!

[vincnetas]

Some stacks are quite complicated, involve various kinds of mobile devices with different app versions, custom hardware, offline functionality. It's just that not all systems are easy to replicate. But that's another problem.