Ask HN: How do you make sure your business-critical software flows work?

Hi HN! We’re a group of senior university students looking into common practices used in the software industry, and we’re hoping to get some insight from people with experience in the field.

How do you make sure your business-critical application flows (e.g. user sign in, payments and revenue streams) work in the software you write, and what are the biggest issues you deal with when making sure these flows work?

Also, how do you typically find out when something does break?

Thanks for your input!