I had that exact experience. I was playing Half-Life (1998) and I learned that you could download the source and modify it (I was not a programmer, just some little kid with a computer). I somehow figured out how to download a free/trial of Borland C++, setup my dev environment, and recompile the entirety of the source code after making a one-character change (changed the friendtype of grunts from one integer value to another) so that they would treat the player as "friendly" and focus on shooting aliens instead. It was a very painful process.

This was before Half-Life: Opposing Force was released.