What's the #1 instant amateur red flag in a screenplay?

Hey everyone, Haley here. I'm wrestling with a new dramedy pilot and trying to polish it up before it goes out. I've been hitting the books again, rereading some craft guides, and 'on-the-nose dialogue' always pops up as a big no-no. I get it, we want subtext, not characters explaining everything. I'm actively trying to cut down on that.

But it got me thinking beyond dialogue. If you're an industry reader, sifting through hundreds of scripts, what's the single most revealing mistake that makes you immediately think, 'Okay, this writer isn't ready'? Is it clunky action lines, predictable plot points, underdeveloped characters, or something else entirely? I'm genuinely curious what screams 'amateur' the loudest to seasoned eyes. What's that one thing that just gives it away?