What isolation actually has to mean
Genuine environment separation means development, staging, and production don't share credentials, aren't reachable from each other using access valid in only one, and don't silently share the same underlying data. Calling something a 'staging environment' doesn't make it isolated — isolation is a property of the actual access boundaries, not the label attached to a deployment.
Without real separation, a mistake made somewhere lower-stakes — a test credential accidentally left with broad scope, a debug endpoint left open in a non-production build — becomes a path into production it was never supposed to have, simply because nothing actually stopped it from reaching there.
The quiet failure mode: production data in a lower environment
A frequent and easy-to-miss violation is production data copied into staging or development for testing convenience, without the same access controls following the copy. The data hasn't become any less sensitive by moving; it's simply landed somewhere with weaker protection around it, which defeats the isolation the separate environment was meant to provide. Synthetic or scrubbed test data avoids this specific trap; a straight copy of production data does not.