What are some common mistakes to avoid when designing a help system?
Procrastinating on the documentation. Documentation can be one of the more boring aspects for many game developers. However, it almost always takes longer if you put it off to do another project. You'll have to go back into the code to recall the exact syntax, what the command was intended to do, etc. Another thing to avoid is a menu-based help system. Menu-based systems create friction for players by requiring multiple commands to navigate to a single file, lots of extra reading, and memorization.