Customer said zone 3 wouldn't come on so I checked resistance at the clock and it read open. Located the valve and turned the zone on at the clock and the valve wire measured .003 so figured it was time to trace. I noticed there was a spare wire at the clock and a spare wire in the valve box so I measured the spare wire. It measured 24.7. I noticed the spare wire only had about 1/16" of wire sticking out of the insulation which I've never seen unless a spice broke. Since no one had been in this valve box I surmised that the installer must've connected the last valve wire and the spare wire to this particular valve since it was furthest from the timer. This way regardless of which wire they tied to the clock, the valve would work. Another time a spare wire in the clock was actually tied the the valve. How it became the "spare" I haven't a clue. Hope this helps someone down the road when you have a head scratcher. As Jim says, measure everything and don't assume.