It was a students' lab, where each device is connected and disconnected 2 or 3 times per day, 5 days per week. The wires get a lot of use and abuse.

Usually the problem is the tip that got slightly loose and the signal is now intermittent or null. IIRC, when we detected the wrong wire we had to put it in a box and the guy in charge of the lab equipment would fix it later.