Post-Production Nightmares: What's the BIGGEST Pain to Fix?

Hey everyone, Ben Castillo here. Currently prepping for a pretty elaborate sequence on an upcoming feature, lots of wire work, explosions, the usual chaos. We're running a few ALEXA Mini bodies with a set of LS 600d Pros for some practicals, and the sound team’s got their 833s dialed in. This project is a beast, and I'm really trying to get ahead of common screw-ups. I've been poring over all the usual best practices, and the list of potential pitfalls is long: clothing rustle, missed RF scans, not getting clean audio takes, mismatched framerates, the works.

My real question for this community, especially those of you who’ve pulled some miracles in post, is this: of all the common mistakes made on set, which one is the absolute most difficult to fix later, and which one creates the biggest cascade of problems downstream? I'm talking the 'wish we'd caught that on the day' kind of mistake. What should I be hyper-vigilant about avoiding?