1. The overlap of the welding plates.
a. The overlap of welding plates means the holes are overlapped, when multiple drilling happens to one position, it would result in the damage of hole.
b. Overlap of two holes in a multi-layer board will cause it scrapped.
2. The abuse of graphics layer.
a. Doing some useless connection in the graphics layer is easily misunderstood, for example, a design for 4 layer PCB uses the circuit of 5 layer PCB or even above.
b. In violation of conventional design, such as, the side of electronic parts is designed at the Bottom layer, and the welding side is designed at the Top layer, these all will cause troubles.
3. The characters are in a mess.