Every PCB design is different. By nature, each design has to be different, as they are all designed to meet different needs. While engineers will follow regular guidelines in designing their boards, and test the boards at various stages to ensure functionality, there is a risk of human error.
In addition to the risk of human error, there are inevitable risks in manufacturing. The manufacture of boards is complex, and there are several opportunities for defects to be included in the PCB boards manufacturing process. An example could be if two metal traces aren’t appropriately insulated, a high voltage may lead to arcing which will destroy the circuit.