[nine_k]

You can test in production by moving fast and breaking things (the clueless guy).

You can test in production by having canaries, filters, etc, and allowing some production traffic to the version under test. This is the "wired" guy.

For many backend things, you can test in production by shadowing the current services, and copying the traffic into your services under test. It has limitations, but can produce zero disruption.

[erhk]

Until you discover that prod has an edgecase string that has the blood mother of cthulu's last name written in swahili-hebrew, (you know it as swabrew, or SHE for short) which due to a lack of goat blood on migration day is one of the ~3% of edge cases that aren't replicated and now youve got an entirly new underground civilization born that is expecting you to serve them traffic with 5 9's and you can only do 4 because of the flakiness.