Most Common Gear-Matching Mistakes?

I've finally come full circle in assembling a system
that I am pretty happy with. All along, though, I've
been wary of pairing up equipment that may not match well with another link in my system.

What are some of the big mistakes that hi fi enthusiasts
make when assembling a system? And I don't mean choosing
a cheap piece of gear, or a poorly-performing brand. Rather
I'm talking about poor synergies between equipment.

Just want to become better informed, because I know it's
just a matter of time before the upgrade bug bites.
Most folks don't take impedance matching seriously. It's just as important,if not more, than anything else.
As an example to Philfreak's responce:
SET with 2Ohm speaker load for instance...

My example is more psychological:
Forcing yourself to like the sound due to the price paid...
To elaborate a little on what Philefreak stated: I believe he's talking about the impediance match between pre-amps and amps (primarily), since source component electrical charachteristics are pretty standardized. But there's more than just impediance. In an extreme example, a passive preamp will work better into an amp with higher input impediance, but it is of paramount importance to also keep the capacitance of the interconnects as low as possible in this case.

You also must match the amp to the speaker correctly. The efficiency of the speaker is a good driver in the choice for SS vs tube vs low powered SET amps. Without the proper power to efficiency you will lose dynamics and low level details.

But the one overriding design goal that I've found to be true is to find the most neutral pieces of equipment possible. It's usually best to not try and compensate for a bright speaker by getting an overly warm or rolled off amp, or a dark speaker by using silver wire.

This kind of "matching" doesn't bring you to neutrality, since you will be losing something at each stage. It can be used as a stop gap measure while you're searching for balance, but ultimately it doesn't satisfy for long.

Relying on other people's opinions, reviews and assigning way too much importance to specs - including impedance matching - without first listening. Sorry, but looking towards these things without critical listening and evaluation in your own listening environment will get you nowhere. I'm not saying you can squeeze blood out of a stone, but a "match" or "mismatch" might not be what the numbers really say; and should be irrelevant if your ears are telling you something else.

It took me months to put together my system - each component was listened to for weeks before I wrote out a check. Yes, I used a dealer and paid retail (huh?) but in exchange I got to audition gear in my own room, got help and advice on setup, which cables etc to try and had the luxury of rearranging, tweaking and evaluating until I was happy within my budget. If I solely relied on specs, then I would have never purchased my 2A3 SET back-up amp to plug into my 6-ohm 86 db speakers with networked cables in a 27 x 15 ft room. Sounds great for my purposes - even though the math says otherwise.