Engineering wants to slow down and fix stability issues. Product wants to ship faster and hit deadlines. Who’s right? Both - and neither. The real question isn’t “should we prioritize reliability or velocity?” but “how much unreliability can we tolerate while still meeting our promises?” That’s your error budget: the quantitative answer that turns endless debates into data-driven decisions. With a 99.9% SLO, you get 43.2 minutes of downtime per month to spend on innovation, experiments, or controlled risks.