About 20 years ago I bought a PS Audio Ultralink II, their totl DAC. I raced home with it from Denver up into the mountains, hooked it up, started a disc and...nothing. No sound at all. I took the top off the thing (like I had any idea what I was looking for), didn't see anything amiss, and put the top back on. It worked perfectly.
A couple of years later, I was in a shop in Boulder, looking at a used PS Audio preamp. I asked to hear it, they wired it up, and - no sound. The salesman was mortified, but I just said, "Try taking the top off and then putting it back on." He looked at me like I was insane, but went ahead, took the top off, and then put it back on. And it worked fine.