What is the worst advice you've seen someone (like from another blog or a writer's book) give a writer?
The absolute worst advice I've seen is recommending that you NOT follow the publisher's submission guidelines.
The guidelines have been created because they make things go more smoothly within the publishing house. You may not like the hoops you have to jump through, but if you want to work with that publisher, you need to jump through them anyway.
The next worst advice is that you don't need to worry about getting your manuscript perfect because your editor will fix it, and as a tangent to that, to purposely leave in something "bad" because editors feel like they have to change something and that will give them something to change and they'll leave the rest of your book alone.
That is just ridiculous! First, if your manuscript is too far from perfect, it will be rejected because it will take to long to clean it up. Second, editors don't have time to fix things that don't need fixing. Leaving in something "bad" is just idiocy and makes you look like an inconsistent writer.
One last piece of bad advice is to look at what's hot and then write a formulaic novel based on the current trend.
This is just so wrong. Formulaic novels are usually boring and will be rejected. And by the time your book makes it through the publishing process, the current trend will most likely be old news.