After enabling React Compiler we removed our manual useMemo calls, and the order table became slower than before. The compiler had bailed out of that component because it mutated an array during render. How we found which components were compiled, fixed the rule violations and verified the result.
A Node.js queue worker restarted a few times a day, dropping in-flight jobs. An analytics call was started without await or catch; when the analytics service failed, the rejection was unhandled and Node.js terminated the process, as it does by default since version 15. How we fixed it.
A refactor passed a class method straight to .then() and our sync client started crashing with Cannot read properties of undefined (reading 'retries'). Methods do not carry their object with them. We compare bind, arrow fields and wrappers, including the arrow-field trap with subclasses.
A routine dependency update moved us to ESLint 9, and lint failed in CI and every editor: ESLint couldn't find an eslint.config file. ESLint 9 uses flat config by default and ignores .eslintrc. How we migrated a TypeScript and React project, kept Prettier out of the way, and avoided the common traps.
Our pricing page listed plans as 100, 1000, 25, 250. Array.prototype.sort() without a comparator converts values to strings and compares UTF-16 code units. We fixed every call site, handled nulls and mixed strings, and added a lint rule so it cannot return.