Have you ever turned on your system, only to find

the sound was totally wrong? Nothing focused, imaging gone, soundstage just disappeared? The bottom end had no impact, the voices were from everywhere, yet when I moved my position slightly left or right, the voices moved in that direction. The music sounded as if it jumped from in between the speakers out into the room very close to my chair. I was going nuts! I repositioned the speakers for the next 4 hours only to find that whatever I tried wouldn't bring back my marvelous sound.
Then I remembered I swapped cables between amp and pre last night. The pre has 2 outputs, one inverted from the other. I plugged both into the right side, one normal, one inverted. What a boob! Anyone have similar mis-steps they would like to share?