“Write every day” is up there. Neither life nor inspiration work on such a set schedule. Plus, if I spent the effort on writing at that pace, then I wouldn’t have much to write about or a perspective anyone should care to read.
But my worst bit of advice received involved a quote. It was early in my writing career. I had quoted a satirical piece as a straight piece and a reader caught it within the first hour or so of the piece running. She notified my editor. The informer was being helpful and as it happened the ease of mistaking satire for the real deal bolstered the argument I had made in the piece. Now here is the bad advice: because of the argument and how early it was caught, my editor recommended we change the piece without adding a note to that effect. I didn’t have access to the piece anymore, so she made the change, but that still bugs me years later. It was a substantive piece of the argument.