The main mistakes/crutches I have lodged in my code–I am starting out–have all been mentioned.
I spent maybe two months working on my first two rooms–to be fair, there were verbs and tables, too–and a big chunk of that was reworking all of the “instead” and “before” stuff into the “check, carry out, report” idiom. It was worthwhile, of course. The code was much easier to read and troubleshoot, and this approach allowed me to better leverage values and kinds.
Speaking of reading: organizing the code has been mentioned, and that has helped a lot, too.
Really, though, and I’m not trying to butter anyone up, the biggest mistake I’ve made is relying too heavily on the handbook and documentation without searching here. It’s no fault of those sources, which are both fantastic, but all manuals make assumptions about the reader’s intent. Since forumgoers express intent in their posts, it’s much easier to tell if this or that tactic is right for your game.
Also: “undescribed” seems like a good idea but it likely isn’t.