We've been replacing ancient electrical outlets at work. Yesterday, we had three receptacle boxes all in a row on a wall. The conduit from the breaker panel ran to the center outlet, the separate conduits ran out of each side of that box to the other boxes. In the past, the left outlet box was on it's own circuit, attached to breaker 4, and the other two were attached to breaker 6. At some point, someone decided that they didn't need two separate circuits, and interrupted the wiring for breaker 4 in the box for the center receptacle, and wired the left receptacle off the center receptacle, so that all three receptacles, left, center and right were powered by breaker 6.
All well, and good, but...
The pinhead who wired it attached the hot and neutral leads for both breaker 4 and breaker 6 to the center receptacle, resulting in three chained receptacles that were powered from both breaker 4 and breaker 6. So, if either breaker was on by itself, didn't matter which, the receptacles had power. If you tried to turn them both on at the same time, they both tripped.
Holy Cow!