He promoted his best engineer.

Six months later, he was sitting across from me trying to explain why that same person had become the most difficult manager on the team. I had seen it before. The engineer who built his credibility on knowing more than everyone around him. The one who got results because his technical judgment was almost always right. The one who earned his promotion because, on every problem that mattered, he was the one you called. The promotion made complete sense to both of them. The engineer saw what he wanted to see. He was already leading the technical work. He already had the respect of his peers. The title was recognition of what he was already doing. Or so it seemed. The leader saw what he wanted to see too. The team trusted him. He delivered. He was the obvious choice. And honestly, there was no one else. What neither of them named was that the job had quietly grown beyond what neither one discussed it to be. As an individual contributor, his value was precision. He knew the domain cold. He had earned the right to be certain, and that certainty is what made him effective. The skill that made him excellent was depth, command, and the confidence to defend his position under pressure. As a manager, that same skill set becomes the problem. The new role required things nobody had named out loud. Cross functional context. Requirements that came from customers, not code. Decisions that involved perspectives he had never needed to consider before. He could sense the edges of what he did not know, and instead of leaning into that uncertainty, he retreated to the territory where he was still the authority. He dug in. He pushed back harder. He controlled what he could control and dismissed what he could not. He complied when he had to. He never bought in. That is not a character flaw. That is what happens when both sides see what they want to see, and nobody names what the job actually became. When a technically excellent employee struggles after being promoted into management, what is usually missing: preparation, support, or something else?