As an aside, some cable testers can't check the first 20 feet or so of a cable. The reason for this blind spot is that a pulse transmitted by the tester will be reflected back to the device before it is entirely transmitted. Thus, the tester can't get an accurate reading. A load may interpret a noise spike as part of a digital signal, distorting the original content of the signal. Common sources of noise spikes include AC lines, telephones, and devices such as radios, microwave ovens, and motors.
Some cable testers test for noise, running tests at different frequencies.
As mentioned, when a current moves through a wire, it creates an electromagnetic field. This field can interfere with signals traveling on an adjacent wire. The twisting allows the wires to cancel each other's noise. The risks of NEXT are highest at the ends of a cable because wire pairs generally don't have twists at their ends, where they enter connectors. If the untwisted end length is too long, NEXT levels can rise to distorting levels.