# Tax calculation regression: first 30 minutes checklist

Source: https://releasefailures.com/incidents/tax-calculation-regression/
Content version: 1.0.0
Verified on: 2026-10-21
Licence: CC BY 4.0

Definition: Tax or VAT is computed with the wrong rate or jurisdiction after a rules or data update.
- [ ] FM-01 Compare the VAT rate on invoices issued since the release with the rate table entry for each affected item.
- [ ] FM-02 Query the database for bookings stored with an empty or invalid tax code since the release and record the count.
- [ ] FM-03 Disable the feature that writes tax codes on overridden prices, or revert the release that introduced it.
- [ ] FM-04 Decide on rollback by checking whether new bookings still receive invalid tax codes after the feature is disabled.
- [ ] FM-05 Write a remediation script for the stored tax codes and run it on a database copy before production.
- [ ] FM-06 Tell affected customers which invoices carry invalid tax information and what guidance applies to them.