I learned a heckuva lot from reading the ADVENT Fortran source code. I was floored when I figured out how it was saving its configuration - such a brilliant idea. And in DOS it could be implemented in about 5 lines of simple C code. (Code size was critical in the old 64Kb days.)

The other huge thing I learned from ADVENT was polymorphism. The comment in the source code "the troll is a modified dwarf" was an epiphany for me.

I forgot to mention, on a floppy disk system, saving the configuration in the exe file made for fast loading of the program, since it didn't need to do extra floppy file operations to load the config.