Let's talk mistakes, what is your biggest? I will tell you mine.
For me the bad mistakes cost the most money. And I am a contractor So I am going to go with a home we remodeled and the plumber rerouted one of the drains and never tied it into the main under the basement slab . So naturally we finished the basement. Drywall, paint , trim flooring, kitchenette, bar the whole deal! Then we ran through our normal QC to find a major back up. Of course the connection that needed to be made was located under our newly tiled floor. About 4k later and 2 weeks of delay you would never know..