Float precision loses cents
Finance flags that a small fraction of ledger rows have `amount` values off by a fraction of a cent — e.g. 19.99 stored as 19.989999999999998, and a few aggregations now disagree with the source by a few cents per million dollars. Started after a Friday schema change. Dashboards: no errors, no failed writes, transaction volume normal. Context: a migration 'cleaned up' the `amount` column on Postgres from `NUMERIC(12,2)` to `double precision` to 'make math faster', and the app's ORM now round-trips the value through a 64-bit float before writing. How do you triage, stop the drift, and recover correctness?
Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.