Failure to double-check basically everything will eventually lead to expensive situations.

A friend of mine is building a house right now. It's supposed to have a chimney for a fireplace. Since the house is built to the latest energy efficiency standards, it's fairly airtight and has a ventilation system with heat recovery. That means that the chimney has to be a so-called air/exhaust chimney (it supplies air for the combustion in addition to transporting the exhaust out of the house, since otherwise the ventilation system might actually suck air into the house through the chimney, which is quite bad if there's a fire burning in the fireplace). The architect did keep that in mind ... however, apparently, this information was lost at some point and the chimney that was installed is a completely different brand, and it's not an air/exhaust chimney. Oops. That'll be an .. interesting ... mistake to fix. It'll require more than just a few lines of code and/or some soldering.