Our cleaning job filled missing countries and lowercased emails, yet the output table still had nulls and mixed case. Chained assignment wrote to a temporary copy, and the warning that said so had been silenced. We fixed it with .loc and turned on Copy-on-Write.
A handful of tests in our Python service failed only when the whole suite ran, and only in some orders. A session-scoped fixture returned a mutable settings object that one test modified. How we proved it with random ordering and bisection, and how fixture scopes, factories and monkeypatch keep tests isolated.
Our CI compared LLM summaries against stored snapshots at temperature 0 and failed about one run in five. Temperature 0 is not a determinism guarantee on hosted models. We replaced string snapshots with property checks and a scored eval.