What's a vendor lesson you learned the hard way?

Everybody who's managed a property or sat on a board has one of these. The bid that came in low and left expensive. The contractor whose change orders quietly doubled the job. The "licensed" guy whose license was inactive the day the claim hit.

What's the one thing you check now that you didn't check on your first hire?

Mine: I pull the COI from the agency directly, not the PDF the contractor emails me. Learned that the hard way after a denied claim. Took exactly one.