Screw is forgotten or snapped: nothing is holding the wires, the resistance of ground connection is too high, switches either don't work or misbehave when you t

Screw is forgotten or snapped: nothing is holding the wires, the resistance of ground connection is too high, switches either don't work or misbehave when you test them before flight. A lot of curse words about servicing the damn thing is heard. Screw gets more and more loose (alternatively, dirt or rust accumulates): less and less current flows through the contact point, more and more voltage is applied to relays. Generally, it is not enough to switch them, until one day another bump changes the position of wires, and ads some more resistance… If something fails, it should fail as a whole. It's analogous to aborting the program on unexpected error versus ignoring it and hoping that everything else generally works OK.