1. Public Speaking. If you can't talk coherently in front of a group of people and sell your ideas, your career will be stunted.
2. Accounting. Accounting is the language of business. If you don't understand double entry bookkeeping, you can't be a manager. You can't run a startup. You can't talk to investors. If you misuse terms like "gross margin" you'll be overlooked as someone not worthy of doing business with. And worst of all, your feathers will get plucked by people who do, and you may never even realize it.
Neither of these are particularly hard to do, but like flossing one's teeth, avoiding them will be costly to your future.