[mmsimanga]

To add it's not just languages. I was once assigned to teach Cognos a BI tool that's been around a while. I was given the instructor's manuals to go through. It was then I realised how incredible sophisticated the software was and I also realised most people didn't even use 50% of the software's capabilities. I have found the same with text editors and IDEs. You can use an editor for five years and still continue to discover new features.

[saurik]

I have been programming for over 30 years now, and one thing I consider myself an expert in is bash... hell: the (original) author of bash is someone I have known well and used to work for, and he considers me an expert in bash (which is probably another example of this in and of itself), and I once spent a couple months writing my own bash-compatible shell replacement for various reasons with him on the sidelines cheering me on.

Well: I have recently been teaching programming to a 10-year old kid, and this morning he told me about the syntax {X..Y}, which expands to the range of characters between X and Y. I have likely typed {0,1,2,3,4,5,6,7,8,9} a thousand times and now I know you can write {0..9}. I was floored and have it on my todo list tomorrow to see if that is a new feature or if I simply somehow missed it in the man page--which I thought I had carefully read multiple times and which I additionally have skimmed many times--consistently for decades :(.