Beyond room tone: What's the biggest dialogue pitfall for beginners?

Hey everyone,

I'm an AD, and on our last shoot, a short film we did on a shoestring with a KOMODO and some Forza 500 IIs, I was really focused on making sure we got clean room tone. It's something I've seen stressed a lot in the guides I've been through, how crucial it is to have for post. We had a pretty green sound crew, bless 'em, and we were trying to keep things organized.

Now we're in post, and the editor is pointing out some consistent issues with the dialogue across scenes, even in otherwise quiet environments. It's not just the room tone; we got plenty of that. I'm trying to figure out what fundamental, reoccurring mistake I should be drilling into new sound ops, beyond just 'get room tone'. What's that one thing that beginners most commonly mess up that really hurts the dialogue track in the long run?

What’s the single most common pitfall for new sound people that compromises their dialogue recordings?