What's the most unintentional monetary damage you've ever witnessed someone cause?

They had a multimillion dollar transit project near where I loved, like $112 million to replace a train station, a subway stop, and a major bus terminal to combine them into a single entity near Washington DC. They projected 3 years from start to finish, but it took almost 7. They had to reroute the entire bus terminal to surrounding streets and parking garages, which was a traffic nightmare. People using the train station or subway had to reroute their walk sometimes up to a mile off their present walk. While doing demolition, they found that the previous bus terminal was on the site of an old gas station which had been improperly sealed off: they just filled the tanks with concrete. Underneath that, they found tons of the the natural mineral serpentine, which naturally contains asbestos. So now they had a biological hazard which they had spent the last few months blowing up with dynamite into the surrounding city. After that was cleaned up and sealed, The got underway.

There were a ton of other mistakes, but when it was completed, they found defects. The superstructure is made of concrete and thus construction specifications were replete with engineering criteria for the composition of the concrete, and its pouring, curing and tensioning. The Inspector General systematically examined 22 project management and control points from the time concrete was mixed until the time it was ready for final inspection. 14 of 22 control points that should have minimized defects were weak or ineffective. Those defects may require recurring engineering inspections, higher maintenance costs, and they could shorten the planned 50-year useful life. In addition, the IG described the risk of concrete falling onto transit-center patrons.

The entire thing was a huge boondoggle costing the downtown untold millions into the future.