We have a three-way switch setup to control our living room light. Our staircase is on the far wall of our living room and one switch is downstairs while the other is at the top of the stairs. The last owner of the home 'completely rewired' the house as it used to have knob-and-tube wiring. However, we've found out since that he only did this in easily accessible areas and anywhere else that he opened the boxes at, he simply pigtailed off of the old wires to feed the new devices.
The problem with the switches was that they both seemed to act as single pole switches in series.. the upstairs switch had to be on for the downstairs switch to work. The switch upstairs was a new three-way switch connected up in that pigtail fashion. The switch downstairs, which we didn't notice until we took the cover off, was a really old three-way switch that was still connected with the original wires (appears it was never modified.) After taking it off of the lines, we were able to see that it must have been fried internally as a continuity test showed that when it was flipped one way, there would be no power to either traveler. However, if it was flipped the other, both travelers would become powered. We figured this was the problem (even though I'm not sure why it would function as it did), added a new switch in it's place, and reconnected the wires on both switches.
The switch that was upstairs was put in place downstairs and we added a new three-way upstairs (not sure entirely why, we just happened to carry it down with us) and so I'm wondering if it could have been fried as well on the inside. As I still don't get why the circuit was acting like two single pole switches. The power fed in through the downstairs switch, went up the walls to the second floor switch, and then went to the light back downstairs and finally fed it's way back to the panel (the neutrals weren't run together originally). I'm not sure where the homeowner tied the old wires to the new ones, but there are no old wires ran into the panel, so he's had to of done it somewhere.
So now, our light works as desired - both switches can cut it on or off in any layout. I gave the above information in case it helps explain any of what I'm wondering about. While we were testing it, I used a non-contact voltage tester at the upstairs switch to see which were hot to insure that we weren't having the same problem as last time. Although the switches work perfectly by visual of the living room light, every other flip of the downstairs switch was showing both travelers hot. Flip it one way and we'd have power to say the left, but flip it the other and both sides were getting powered.. as far as the voltage tester showed.
The downstairs switch would work by diverting the power from traveler to traveler, while the upstairs switch would work by picking which side to accept power from. So if both travelers are getting power, then why would either switch still work at cutting the light off? Possibly just a small amount of power bleeding through?