Most Costly Location Mistake for Beginner ADs?

Hey everyone, I'm Simone, a 2nd AD, and I'm always trying to streamline our location scouting and management to avoid headaches down the line. I'm working on a lower-budget indie feature right now, and while I feel like I've got a decent handle on the basics (detailed photos, good communication with the PM) I'm also acutely aware of how quickly locations can chew through a budget if something goes sideways.

I’ve been going over some resources that mention common location mistakes for newer ADs, like not doing thorough site visits or being vague about crew size. But if you had to pick one single, most expensive location mistake a beginner AD is likely to make, what would it be? I'm talking about the kind of screw-up that sends the budget spiraling or tanks the schedule. Trying to learn from others' hard-won lessons here before I make my own!