A `Mutex` is a fairly common concept in concurrent programming. Yet, despite that, **cats-effect** didn’t provide one until recently. The reason was that they are conceptually equivalent to a `Semaphore` of a single _permit_. And, actually, the original implementation was just that. However, that changed with the `3.5.0` release, which introduced a brand-new, roughly 20-25% more efficient implementation. In this talk, we will cover the iterative process that we followed from the very original implementation up to the final one that was shipped. Yet, the main idea of the talk is not just to relate the story, but to share the insights that we learned from the experience. Ranging from pointing out how annoying race conditions can be, through highlighting the differences in the imperative VS functional mental models, up to the biggest cliche yet most valuable lesson from all-time science: “standing on the shoulders of giants”.