The failure mode of primary concern is an open LED. One of the biggest enemies of LED devices is heat. Also, temperature cycling and mechanical vibration can compromise the tiny bond wires that connect the LED die to the package terminals. If a bond wire fails, the LED will go dark and all of the remaining LEDs in the string will also go dark because no current can pass through the open LED. Note that the full compliance voltage of the constant current source will appear across the open LED. Therefore, a single LED failure causes an entire string to fail, resulting in product returns, or additional warranty expense for the LED lighting manufacturer.