Every bug found and fixed — with the root cause, not just “fixed”.
“It's fixed” is a claim, not evidence. Without the root cause written down, the same bug comes back wearing a different hat.
A running log of every defect: error code, what broke, why it broke, what changed, and how it was tested. Each entry carries a status — OPEN, FIXED or VERIFIED — and VERIFIED means someone actually looked at it live, not that the code was merged.