Beyond Room Tone: What's the BIGGEST audio pitfall for beginners?

Hey everyone, Terrence Cole here. I'm a working DIT, usually wrangling footage from an ALEXA Mini or a V-RAPTOR XL, and I'm dipping my toes deeper into understanding all aspects of a shoot since I'm transitioning to supervising more. I've been doing a lot of reading and one thing that always comes up is the importance of room tone, neglecting it is apparently a 'costly mistake'. I totally get why that's crucial for post-production and cleaning up edits.

But that got me thinking: aside from the lack of room tone, what's the single most common, glaring mistake beginners make that utterly compromises their dialogue track? I'm talking about something that's difficult or impossible to fix in post, even with the best plugins. Is it mic placement? Gain staging? Using the wrong mic for the environment? I've tried to be diligent about monitoring during shoots, but I'm curious what the pros here see most often from newer folks.

What’s that one pitfall that consistently ruins dialogue, right after skipping room tone?