Major mistakes made in assembling your audio rig

Forcing tab "A" into slot "B".

This major "mistake" turned out to be a major lesson that would help me avoid other mistakes in the future. Many years ago, as a young budding audiophile, I made my first brand new (full retail) purchase, the AR SP9 preamp. After reading some rave reviews I was excited at the prospect of owning my first AR piece. I went to a local dealer where I heard it in what is still one of the most memorable systems I have ever heard: Snell Type Aiii, VTL, Roksan/Koetsu, SP9, MIT. I brought it home only to feel great disappointment because, even after a long break in period, it made (more accurately, it let) my system sound bleached out, thin and threadbare and just plain awful. After several days I took it back to the dealer who had agreed to issue a store credit. I thought that perhaps there was something wrong with this particular sample. In order to test it he inserted it into the same system as before and, once again, the sound was glorious. That was the moment when I understood the importance of system synergy and the idea that some gear simply does a much better job of getting out of the way and revealing the sound of the other components in the system. A great lesson. The store credit resulted in, among other things, my first vacuum record cleaner; another great lesson.

my way to avoid a major mistake was to find speakers that made emotional music and listen to them several times before purchase. Try to hear as many speakers as possible so you are reasonably confident in long term satisfaction.
since speakers are the one component that seem to have the biggest influence on a system's sound, start by choosing your speaker first.

In my toughest room acoustically, which is my wife's sunroom, it took me a long time to get something working well in there. Always either too much bass, too bright, or both.

Finally I ended up with a good solution currently in place using my Triangle Titus XL monitors on lower stands, only about 12" off the floor. This nails it pretty well. Higher up these were always too bright and not enough bass, whereas Dynaudio Contour 1.3mkII monitors or my smaller OHMs had too much bass.

I was tempted to try/buy other speakers to attempt to solve the problem on several occasions, but resisted the urge, determined to make something I already had that was good work.

It's always fun to play with new toys though, so I suppose I missed out on that, but I already have more than I can listen to as much as I would like, so can't complain.

The biggest mistake was trying to balance out a flaw in a component with a component that had the opposite flaw. I ended having to get rid of both components and replace them with components that did not exhibit major shortcomings.