I consider overhoning as sharpening beyond what is needed, which leads to reduced or bad performance with the blade. For instance, a butter knife should be dull so that it will shear the butter, causing it to stick to the knife, and spread the butter evenly. A sharp butter knife is overhoned and will slice through the butter causing it to fall off of the knife and make a scraping sound on the bread as the blade cuts the butter and the bread instead of shearing it to spread.

As for HHT, I only use it as a rough guide that the bevel is set. A fail for me would mean that the bevel isn't set, so under honed.