There are two abnormal and rare 3-way systems that may be encountered. They are called by various names (California, Hollywood, coast, farmer’s, French, Chicago, Carter, lazy Susan, lazy neutral). The names are confused between the two kinds. One is illegal and presents possible shock danger. The other, which seems allowed by code, is so different in its concept that electricians prefer to avoid installing it. What I am saying about 3-way systems is not meant to apply to either of these. Basically, the illegal one attaches a hot permanently to one traveler and a neutral to the other; the commons each extend from their terminals to the light fixture. The legal one likewise attaches a hot to one traveler but the leg (to the light) to the other; another wire runs between the common terminals. Here is a diagram of these Rare 3-ways.

an illegal three way in OUR HOUSE? how could THAT happen?