I have a cable tester made by Ideal. It'll tell you almost anything: short, open, which wires are crossed, ect. However, there's one flaw that I found. One time I inserted the wires in the connector while holding it upside down and on the other end, I inserted the wires in the connector the right way.
When I tested the cable, each end passed when tested individually. So I was more than a little confused when it didn't work. It's always best to test both ends at the same time. That revealed the problem.
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