I don't think anyone actually answered the original question and I have the same one. I want to know when you become responsible for someone's mistakes.
I am a CFA and a formerly much-registered financial analyst. I am not a lawyer or an accountant and currently have no clients other than my spouse & our businesses. So consider this a somewhat-educated opinion.
Answer: It depends.
I don't mean this to be a snarky answer - really, it *does* depend.
Long story short: if you are serious about giving specific investment advice to clients (rather than seminars/life planning/etc.), try calling someone who is doing what you want to do in your state (maybe in a different area so they feel more mentor-y and less competitive) and ask them about it. And depending on what direction you pursue, your state regulatory bodies and a lawyer. If this is an area that interests you, there certainly is the potential for side-gig$
Long story less short:
Each state regulates this separately (fun, eh?). How well you document things could make a difference. So could the nature of your advice-general financial planning is a different kettle of fish than specific buy/sell recommendations. How you get paid makes a huge difference. If you are an employee vs. an outside advisor it makes a difference. Who the client is can make a difference (there are different legal standards for the basis of advice to individuals vs. to companies, and yet another standard for advice to pensions). How much money the client has can make a difference in some cases...Whether or not you are liable, the amount the client loses can make a big difference as to how motivated they are to sue you.
Financial planning is typically less regulated than investment advising. With investment advising, the major requirements for SEC registration are tied to being paid by commission - getting paid for your advice directly (rather than getting paid through fees tied to the transactions of your advisees) makes a regulatory difference. (There are also separate state requirements for investment advisors.) Not sure if it is still the case, but used to be if you were a CPA or a lawyer, you could advise your existing accounting/legal clients without taking the registration exams.
Within state laws you could always put together a client contract that addresses or limits some of your risk - but if the contract is too one-sided, don't expect clients to sign it. And you'd want a lawyer to review, typically the client can't just sign away your liability without some trade-off.