We discovered a patch of black mold in the basement of this house, and we conferred with the owner before running the blower door test, since the blower has the potential to spread the mold. We ultimately decided to go ahead with the test because there were no visible air leaks on the far side of the mold (and the lack of ventilation in this location likely contributed to mold growth, since mold likes stagnant air), so the benefits of the test outweighed the risks on this occasion.
The electric bills were quite high year round, and the culprit turned out to be the air handler (furnace blower), which the homeowner was using year-round to circulate air. He thought this would save him money, but on the contrary, it was a very expensive way to get poor ventilation. We recommended adding a heat-recovery ventilation system.