A CSS preload optimization shipped, ran on every page load, and did nothing. Not "did nothing dramatic" — did nothing at all, silently, for the length of time it took someone to notice the preload tags weren't matching anything a browser would actually reuse. The fix was two characters in a regular expression.
A script that was moved to run earlier on the page started throwing an error it had never thrown before. The fix that had made it "run early" and the bug that made it crash were the same change.
A theme library definition and its JavaScript file are both still sitting in this site's codebase right now, fully written, fully functional, and never loaded by anything. Not disabled, not commented out — just quietly disconnected from the one file that would tell Drupal to load them.
The same inline JavaScript snippet got moved to a different delivery mechanism three times in thirty minutes. Two of the three moves were blamed on an "& encoding" bug. Only the third move actually fixes the kind of bug that explanation describes — and checking the file at each step shows the first two blame something that isn't there.
Projectile ships with sensible defaults for switching projects and listing files, and most of my config leaves packages at their defaults unless something specific needed to change.
Moving a template file from one directory to another shouldn't change what it renders. I moved html.html.twig to templates/layout/html.html.twig for Drupal 11 compatibility, and the rename quietly deleted an AdSense script tag and three Google Fonts links that had nothing to do with the move. Nobody noticed for hours.
My Emacs config loads two separate linter frameworks in the same file, both fully configured, both actually running: flymake and flycheck. They're not doing the same job for different languages, and neither one is dead configuration left over from a migration that didn't finish.
Seven packages in my Emacs config are fetched with package-vc-install instead of the ordinary archive-based use-package foo :ensure t: auto-package-update, powerline, centaur-tabs, lsp-mode, php-mode, dap-mode, and smartparens-mode, each cloned straight from its GitHub repository
Two different lines in my own Emacs config call the exact same function twice, in the exact same file, a few lines apart. Neither one breaks anything. Both are worth understanding, because they demonstrate two different things about what use-package actually guarantees — and neither one is what I assumed before I went looking.
My Emacs config has three completion frameworks installed and configured: auto-complete, company, and corfu. Only two of them are actually doing anything.