Four patterns we removed from real code: a singleton holding hidden state, an abstract factory with one product, an observer that leaked memory, and a strategy hierarchy for two if statements. What each cost us, and the signals that tell you a pattern is worth it.
Not a list of slogans. One pull request from our billing code, the review comments it got, and the five rules that held up after years of reviews: names, function size, error paths, comments that explain why, and boring control flow.
A customer import that took seconds in testing ran for over an hour on a real file and hit the worker timeout. The cause was a membership test on a list inside a loop. How we found it with a profiler, measured the fix with timeit, and guarded against it coming back.
A new exporter subclass overrode one method to add a feature and silently bypassed the base class check that kept amounts non-negative. Negative invoices reached a partner. Why inheritance made this easy, and how composition closed it.
We copied a config object with spread before customising it per tenant. The nested objects were still shared, so one tenant's override leaked into everyone else's defaults. How we found it, and the copy rules for JavaScript, Python and Go.