those can cause big problems too (power lines, grounds etc.), I had one last year, loosing one leg of the input power intermittently or partially took a bit of doing to find that one!
What I was referring to is a ground circuit in your machine or on a circuit board, their can be several, and they arent easy to find unless its obvious (burnt connection, discoloration etc.)
Vibration can be a big cause of this, loosening lead connections etc.
I had one on the X stepper motor driver recently, it was relatively easy to find, it was missing steps and odd noises occasionally, luckily it got bad enough that I found a burned contact in a connector, found it visually.