As a software developer well into my second decade of professional experience, I maintain a small number of technologies at what you might call an expert level. These technologies shift in and out of focus depending on what my current projects are.
When I complete a project and don't use the technology for more than a year or so, I've found that I forget all of the nitty gritty stuff and remember all the big conceptual stuff.
For example, I recently returned to Java after several years of disuse. All the bit conceptual stuff that was really hard for me to pick up initially, like polymorphic behavior, multithreading, etc., was still there. The easy but nit-picky stuff was all gone. I'd forgotten when boxing happens and doesn't happen, the behavior of equals in reference vs value types, even where I'm supposed to put certain syntactic elements. Simple questions on StackOverflow to the rescue!
As another example, I did a large project involving SVG in the early 2000's and got to the point where I knew as much as there was to know about it. I recently did a quick one-off project that utilized SVG, and I found that I'd retained the big conceptual ideas, such as the behavior of the coordinate system, the hierarchy of shapes, viewports, groups, etc., but I'd totally forgotten a huge laundry list of practical nitty-gritty things about actually making an SVG experience work.
In the Java example I was embarking on a large project, so I hit the books and re-taught myself to fish again, because it was quite worth my time investment to start from the fundamentals and work my way back up. In the SVG example, I literally just wanted to do something in an afternoon, and I knew SVG could do it, and I wasn't going to do any SVG work after that. Hitting the books and teaching myself to fish in that scenario would have been a waste of time. So I plowed through and was helped immensely by the simple-question simple-answer Stack Overflow scenario.
Then there's a whole list of technologies that I really don't have the brain-space to keep abreast of, but I still need to use. For example I am not an expert at shell scripting, but on occasion I need to write one. Back to Stack Overflow and the simple answers to simple questions.
Before Stack Overflow I wouldn't have been in the dark--as a long-time Internet community member, I would have gone through the usual: find the right community with the most helpful people, hope the community has a search engine or is well indexed by Google, read through long lists of replies without a voting system or assessment of quality, rinse-repeat. Stack Overflow speeds that process up immensely.