HDMI cables have a lot to deliver-billions of bits per second, to be precise. What does it look like when they fail to meet the challenge? Cable-induced video artifacts can be as small as color banding (loss of color gradations), pixel dropouts, "sparklies" popping intermittently on the screen, and "solarization," where the gradients of color are not accurately produced.
In severe cases, there is "noise" and "snow" and streaks that occasionally blast across the screen. In the most severe cases, the picture will black out altogether. Customers have reported many instances in which picture quality was poor, and replacing their generic cable with a higher speed cable for HDMI™ solved their problem.