Unless I’m forgetting something (always possible), there are only two things you can do wrong when writing fiction. The things you can do wrong are being incoherent and being boring.
Those are both big categories, however.
“Incoherent” includes everything from bad grammar and sloppy pronoun antecedents to absurd character motivations and plot threads that are left dangling. Failure to set the scene properly, so that the reader can’t tell what’s going on, is also a way of being incoherent. Failing to include dialog tags for more than three utterances, so that the reader can’t keep track of who is talking, is incoherent.
“Boring” includes being long-winded to no purpose, using hackneyed material, and writing in a pedantic or pedestrian style. If your characters all speak with the same voice, that’s boring. If you’ve included a 20-page Prologue that doesn’t really need to be there, that’s boring. If you’re writing science fiction, going into detail for pages about your imaginary technology is going to be boring. If you’re writing fantasy, zombies and vampires are boring.
Wizards are almost as bad, but having recently finished an epic that includes wizards — well, I’m not proud of the fact that I used wizards, but given the nature of the story, there seemed to be no practical way to get rid of them. A story infested by wizards runs a grave risk of being boring.
Now let’s flip this idea on its head. If your novel or short story is coherent and interesting, you’ve got a winner. That’s pretty much guaranteed. You can switch viewpoint, indulge in authorial intrusions, or do almost anything else that aspiring writers are urged not to do, as long as your story remains coherent and interesting.