After we set "type": "module" in package.json, jest.mock stopped replacing our mailer and tests started calling the real SMTP client. Static ESM imports are linked before any mock runs. Here is the mechanism and the fix with unstable_mockModule.
Our reading progress bar stopped at 92% and saved scroll positions were always slightly behind. A hand-written leading-edge throttle dropped the final scroll event of every gesture. We added a trailing call, tested it with fake timers, and compared requestAnimationFrame and scrollend.
A billing test passed for weeks and failed this morning, 31 January, for every engineer at once. JavaScript's setMonth rolled 31 January into 3 March, and the test read the real clock. We fixed the arithmetic, injected the clock and added property tests over every day.