Major mistakes made in assembling your audio rig

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.

Biggest mistake I ever made was going cheap on a digital scale to set up an expensive cartridge. Some cheap $10 weighs anything scale with a flat pan, instead of a $80 one specifically designed for stylus'.
Long story short, I snapped the cantilever setting up a brand new $5000 cartridge. 8 years later, I still have nightmares about that day....

"Major mistakes made in assembling your audio rig"

For those married or with someone - building it in complete
isolation of your spouse for fear of them knowing what u
spent. If something happens to you - it leaves the family in
a quandry.

You can break a few rules imo.
Its ok to sneak some stuff in .... even major stuff - the kind
that a value can be put on easily - i.e. by a dealer.
Just make sure when you sell something - they see it going out
the door.
this is important. you gain points ....
Include curtains in your room - they help with sonics .....and
to hide gear.