Model confidently stated wrong weekday for a date (unverified derived fact)

Environment: Claude Code desktop app (macOS), model claude-fable-5, 2026-07-09

What happened: While assisting with a deadline-sensitive government form, the model wrote "Tuesday, July 15" referring to 2026-07-15 — which is a Wednesday. The date itself came from a source document the model read correctly; the weekday was added for fluency and never computed.

Why it matters: An unverified derived fact sitting next to verified facts inherits their credibility. In deadline/payment contexts a wrong weekday can cause real-world harm. Every sourced number in the session was cross-checked and correct; the one failure was the detail with no source.

Expected behavior: Derived facts (day-of-week, arithmetic, date spans) should be computed (e.g. quick calculation or date command) or omitted — not pattern-matched.

Impact: Caught by the user; damaged trust in an otherwise fully verified workflow.