I can't tell you a common one, but I can tell you a weird one. We wired this big box store for fire. It had an analog addressable fire system. When we put the loops on line and BIG problems. We finally got them sorted out but it was an eleventh hour deal with 14 guys pulling their hair out, trying to get the CO so the store could open on time. One of the strangest problems was when you would isolate a ground to a particular device and pull the horn/strobe AV out of the back box and the problem would go away. Push it back in (still not touching anything) and the ground would appear. To make a long story short it turns out that the wire itself was to blame. When you'd strip the wire you noticed that the insulation on the inner jackets was nicked at even intervals as far back as you stripped the wire. Since it was asymmetric with respect to red and black if the wire was stretched out it was normal. When coiled to fit in the box the inner wires would touch and a ground fault would appear. The wire manufacturer owned up to the problem (after some early crawfishing) and admitted that they had a bad run. They made it right to the tune of more than 10 grand so I won't mention names.
16 years ago