In terms of cables, the normal cause for EMI tends to be because the cable ends up working like an antenna and starts to simplify the signal being transmitted inside the cable.

Usually EMI, or electromagnetic interference is what's being discussed, but EMC is about how a device works and interacts in an environment so it doesn't cause EMI. The four groups tested 30 coaxial antenna cables and 30 HDMI cables, of which only 11 percent of the antenna cables met the manufacturers declared attenuation and and only 10 percent of the HDMI cables met an acceptable EMC quality of at least 50 dB coupling attenuation. The German Federal Network Agency, together with Agentschap Telecom Netherlands, BAKOM Switzerland and Elsäkerhetsverket Sweden have tested consumer HDMI and coaxial antenna cables and come to the conclusion that many of them don't meet regulations when it comes to Electromagnetic Compatibility (EMC).