Have you ever turned on your system, only to find

Tmorosk, We would think so. Qualified (licensed) does not necessarily equate to competence.

How can a qualified electrician be so careless . It's a very simple procedure .

Similar experience as Mihaitaa. I have w4s monos and was fiddling around back of one and accidentally hit the xlr/rca switch to rca (am using xlr) w/o realizing it. I actually brought the amp in for inspection (i'm local) since all of a sudden no sound was coming out of the speaker, and i thought I had fried the amp somehow. Down the troubleshooting list they finally got the to the xlr/rca switch, and flipped it back to xlr. Everything fine, except me feeling like a dumb*ss

Umm, yep. I used to own an all Linn system that included a Klout power amp. The Klout had multiple sets of RCA outputs for daisy chaining, to facilitate active multi-amp systems. Linn decided to put these RCA outputs right below the main set of RCA inputs. You guessed it, one time I plugged the main pre-amp outs to the amplifier outputs. The resulting sound was a dull summed mono. Took me two days to figure it out, Duh.

My W4S ST-500 just arrived from its previous owner. Gave it a cursory inspection, hooked it up and... the image is off center. By a significant margin.
I employ the balance control of the W4S DAC2 and it brings it back, but still doesn't sound right and why should I have to employ the balance control? I didn't have to before, with the other amp...
Check the interconnects, OK.
Check the speaker cables, OK.
Start sweating a bit. Well, you bought used, here you go, floats in my head. Now what?
Pretty pissed I turn everything off and go to pick up my daughter from her dance class. While driving my mind keeps searching for an explanation and the bulb comes lit. There are two switches on the back of the power amp, selecting XLR/RCA inputs. The previous owner had them on XLR, I switched them on RCA, for my XLR cables didn't arrive yet, maybe there is something there...
And sure enough, when I came home and checked, one of the switches was only halfway switched. Pushed it completely and everything was just fine.
Phew!

My tube monoblocks have a switch on the front panel that inverts phase.

My adult kids use the system occasionally and don't always remember where the power switch is. That's all you really need to know -- that and Murphy's Law. But it took me a bunch of listening sessions to figure out where the trouble was.

I had an electrician put in a new 20 amp circuit he had the polarity out of phase. It pretty much made a mess of the sound. It was about 13 years ago at this triplex that I used to own. Unfortunately he wired it up at the box using the existing outlets and wiring just put on their own circuit. It took me three days to figure it out what was going on. once I put my system on extension cord to the kitchen outlets, it sounded like its old self again. I then bought a plug tester. That is when I found it out of phase and with a floating ground. I now always check to see if the polarity and the outlets are now wired correctly when I set up any system. It is a great tweak to make sure you have good power. I stopped payment on the check to the first electrician an hired another electrician who came out and fixed the polarity plus having to re-do the whole thing right. Once my system was on the new and corrected dedicated 20 amp circuit it sounded better than it ever had, and it was a huge improvement.