OK. This one had me scratching my head the other day:
Testing a Fender Passport system. Complaint was "one output channel dead".
I hook it up using the supplied speaker cables, and sure enough, no output, but BOTH outputs are dead. I check the speakers and cables with another amp and they're all fine. I check the low level output of the passport into another amp and that works as well.

So, assuming the power amp is bad, I order a new module from Fender (unit is under warranty). Something still feels funny about the whole thing, so I test it again with my own speaker cables, simply because the ones supplied with the unit are already coiled up and put away (I already ruled them out as a problem, anyway).
The thing works fine. So, now I'm thinking "intermittent". Power up and down numerous times, whack it with my fist, etc. The thing ALWAYS works now.

I pull out the supplied speaker cables again to try to recreate the original problem.
Suddenly, one channel is going in and out of "protect" a couple times per second. This is different. Power up and down a few times. Sometimes I get the stuttering channel, sometimes I get no audio at all. OK, that recreates my initial observations.

Pull the speaker cables and test them. Cable tester says they're ok. My tester senses intermittents with a latching LED, so I pull on the entire length of both cables trying to cause an intermittent connection. All solid. Test both cables with my DMM from end to end and across tip to sleeve. All ok.

Plug the speaker cables back into the amp...instant failure. So I go round and round testing the amp, the speakers and the cables individually. They all work perfectly by themselves. Eventually, I realize that it's always the same speaker cable inserted when the stuttering channel symptom occurs. If I pull that cable out of the equation, everything always works as it should. I test that cable every way I can think of, but find no fault. Finally, I'm about ready to chuck the cable and supply a new one when I decide to compare the 1/4" plugs between cables. The barrel on the faulty cable is about 2mm longer than the barrel on the good cable! Aha!
So I try the cable again, but don't plug it in all the way, it works.

What was happening was that since the barrel was too long, the part of the output jack in the amp that contacts the tip was actually coming into contact with the sleeve on the speaker cord plug, and shorting the amp output. So I put two new plugs on the speaker cord, and called Fender to cancel my parts order!

Just thought I would share this here as an example of how the simplest problem in certain circumstances can really lead you around in circles sometimes. That bad cable worked in EVERY situation I tried it in, except for being plugged into the Passport output! Argh!