I've been replacing all the outlets in my house (built in 1949, wiring is newer than that) with white outlets to please the SO. I came across an outlet where I had to shut off two circuits to get the the plug not to be hot.
After doing some reading, it seems like some of the likely possibilities are
1) mis-wired breaker-box
2) possibly the two circuits are connected in a junction box somewhere.
The second seems likely, as almost every outlet has 4 wires, two hot wires and two neutral wires, and it's easy to discern the pairs. In general I've just been wiring them like they were, not taking the tabs off the outlets because they weren't off the old outlets.
After reading about this, I realized that if any two of those pairs of wires are on different circuits, I'll have joined the circuits (at least I think that's what I got out of it).
My question is, if there are four wires, is there any reason not to break the tabs on every outlet I'm wiring?