It was almost always performance stuff. I'd get stuck with stuff like "Oracle's cluster database sucks on our systems, can you find out why?" and then you have to go figure out where it is slow (find the hot spot) and then figure out why we were slow.

That meant keeping all of the hardware in your head, the kernel in your head, and the app in your head. If you didn't have all that state it was super hard, maybe impossible (for me at least), to figure out the problem.