I'll get things going.
One piece of terrible marketing advice I vividly remember was on a blog for freelancers, where a guy pontificated that there wasn't a single freelancer who had built a reputation without blogging. Therefore, you must blog.
My response: Hello! I'm a successful freelancer (30+ years) by any measure of success and I do not blog.
I wrote up a long reply to this post explaining why I don't blog, why blogging might not suit certain people and what one could do instead.
They wouldn't publish my reply! They said it was too argumentative. Duh... So I posted it on my own website. (You can read it here.)
Besides the fact that the advice was based on a provably false assumption, why else was it bad advice? In my view, just about anything in marketing phrased as a "must" is dangerously wrong. Almost always, there are alternate ways to achieve a given result. Usually there are many, many ways to achieve the result, and when you're a lone worker, you get more done with better results when you choose marketing tactics that are a fit with your lifestyle, values, preferences and personality.
Blogging isn't right for everyone. And you can succeed without it.
All right now, what's your example of lousy marketing advice from a reputable source?