“On March 2, 2021, we received a report via our support team from a user who, while using GitHub.com logged in as their own user, was suddenly authenticated as another user.”
A performance rewrite introduced a thread safety bug that handed one user’s session to another. GitHub only found out because a user reported it. Then a second user did. The fix was logging out everyone on GitHub.com. Rare is cold comfort when the bug is waking up inside someone else’s account.