Beyond 'On-the-Nose': The BIGGEST Amateur Script Red Flag?

Hey everyone,

I'm diving deep into a new pilot right now, trying to make every word count. I've been really conscious of avoiding clunky, 'on-the-nose' dialogue, which every screenwriting guide flags as a major no-go. I'm endlessly rewriting lines, aiming for subtext and character voice, often reading them aloud to catch anything that feels too expository.

But it got me thinking: if 'on-the-nose' dialogue is a common error, what's the single most revealing mistake in a script that immediately screams 'amateur' to an industry reader? Like, the one thing that makes them close the script after two pages, regardless of the premise?

I'm genuinely trying to level up and want to tackle the foundational issues first. What's the biggest sin, in your experience?