An environment that copies production as closely as you can afford, where a release runs before real users get it.
It catches what shows up only when the pieces run together: a bad setting, a missing variable, a migration that won't run. It can't show what only real traffic and real data reveal, and a pass there can feel like more proof than it is.
Say it to your agent
“Deploy to staging and run the smoke tests against its URL. Then tell me what staging can't show us: it has 200 rows and one user. List anything that needs real traffic to prove, and we'll watch for that in the canary.”