Like edamico 11 says, a lot of times you will be thinking too far out, assuming the worst, and you will not see the simple and obvious.
A perfect example, I was called to a ranch to trouble shoot a remote controlled roping chute that was working intermittently. A previous sparky had been there twice, and had even installed a new, costly circuit board. The problem persisted, and the sparky threw up his hands and told the customer they would have to find someone experienced with roping chutes.
Even though I had never touched a roping chute in my life, they called me out to try my luck. The problem turned out to be a simple power strip, the kind you use on your computer. They had set one up inside the machine to plug in the LV transformer and some other stuff. It had a loose connection where the power cord was soldered in.
The circuit boards and electronic gizmos scared the other guy off because he was thinking too hard.