PCB signal coupling can be a problem.

The article discusses the areas to consider when printed circuit board (PCB) traces are in close proximity while trying to combine analogue and digital signals on the same board. It mentions the need to pay attention to where two traces cross on different layers and on where they run parallel on the same layer. It notes the creation of a parasitic capacitor on one's board if two traces are in proximity. It also emphasizes another scenario which can create board parasitic capacitance.