The situation
Early in my career, on a Microsoft support contract through Convergys, I worked at the top support tier for Windows XP: the Level 3, R&D desk. The role started with a month of Microsoft training on troubleshooting the OS, and within a few months I had become the go-to: the cases Level 1 and Level 2 couldn't solve came to me.
What I did
The work was root-cause diagnosis. I traced what had actually changed on a machine, using Event Viewer and Microsoft's knowledge base, both the public articles and the internal ones, to pin down corrupted registry entries, bad drivers, and damaged system files. I also became a frequent contributor to that knowledge base myself, writing my own KB articles on registry and system fixes so the next person could solve it faster.
What I take from it
It's the earliest sustained example of a pattern that has followed me since: being the person hard problems escalate to, and solving them by finding the cause instead of guessing at it. The same instinct turns up later in reading firewall logs and tuning monitoring.