Beyond 'on-the-nose' dialogue: What's the quickest amateur tell in a script?

Hey everyone, Marcus Johnson here. I'm deep into rewrites on a TV pilot, a sci-fi thriller, and I'm really trying to elevate the writing. I've been focusing heavily on subtext and avoiding the dreaded 'on-the-nose' dialogue that so many guides flag as an amateur mistake. I think I'm making progress there, trying to let character actions and reactions speak volumes. However, it got me wondering about other, perhaps less obvious, red flags. What's that one thing a script does, or fails to do, that instantly screams 'newbie writer' to an industry professional? Something that immediately makes you put it down, even if the premise is intriguing?