These are the first few that came to mind. I'll try to continue updating this list as I think of more
  1. Writing things the can never see nor hear in the movie
    There's a blurry line here and a lot of misunderstanding about what comprises "things the audience can't see". I actually think describing what a character is feeling, while we see a close up on their face can be potentially helpful for an actor. But what isn't fair is putting too many asides meant only for the reader "and that was the last time he saw his wife alive" is the classic Stephen King trick and is a total cheat in screenwriting. At most you get one per script. At most.
  2. Using language and description clearly meant to create association with a recent popular movie
    This is a weird one, and maybe it's more of a problem in the world of horror, where I do a lot of my work. But I've seen new scripts, that sneak in ways to use titles of recently successful horror movies, "she gives him an INSIDIOUS look" is a really terrible example that is not far off from real stuff I've seen. I guess they're hoping to hypnotize or trick an exec into buying their script. Or maybe it's some lame version of a wink? I have no idea but it sucks.
  3. Describing the attractiveness of characters for no reason or bad reasons
    "Pretty but doesn't know it" is a cliche now but I've come across much more offensive versions of this problem. without getting too gross, one script involved detailed description of the sexual attractiveness of a 15yo girl, who was violently raped in the following scene.... (Yes this was an actual script) I doubt I need to explain, but this made me feel complicit in the horrific acts the writer described in a way that made me HATE him and the story he was trying to tell. Didn't finish reading.
  4. Writing with an almost willful ignorance as to how movies are made
    I think William Goldman cites this in one of his books, using the example of a script that opens with a stampede of camels running through Central Park. The point is you need to write things that people want to see accomplished on screen and if they scoff at yr ideas they'll be less likely to try. There are different schools of thought on this but I don't think tying to think like a producer or a director has to limit yr abilities as a writer.
  5. "Write what you know"
    This is a common and misunderstood piece of advice. Many people assume it to mean "write about things that look like yr life" but I disagree. I've still never once written about a 30ish married screenwriter in LA and yet all my scripts come from "what I know". Because I take it to mean "write stories and characters that come from emotional truths you've experienced". This definition allows you to be more broad in yr thinking and yet more specific and real at the same time.
  6. Only having an idea for an opening sequence but writing a 100 pages just to see
    I think we've all been there. But unless yr The Coen Brothers it rarely works out. You might get lucky with some bit of inspiration but in general it's better to explore a story in an outline, and know that it works, before sitting down to write.
  7. Writing something because you think it can sell.
    Don't do this. I don't see cynicism rewarded very often. There are a lot of movies released that I think are stupid garbage but the people who made them certainly didn't set out with that as the goal. Don't write in a genre you hate just because you think it'll be profitable. Write things you love and hope you can connect with likeminded people through the work.