I mean... if you think you know how something works in the beginning and then really dive into it, you always end up thinking "how could I say that I knew something about this before I learned all this?".
You define that you know everything about for example a programming language and environment after having worked with it several years. Then you get into a series of problems that cause you to open your books again searching for a solution, and wow(!)... you learn something new. After several problem solving tasks you have learned so many new things that you just laugh out loud when you think back on the time when you thought you knew everything.
Isn't this ironic? Doesn't the process of learning ever end [rolleyes]? Are humans constructed to learn all their lifes about everything and everybody? I think so. Why else would the term "science" exist?