The two most common mistakes are bass and treble

OK, so I know many of you will have a knee jerk reaction to that with something like "well you've just covered most of the spectrum!" but I mean to say more than what I can fit in a headline.

When first purchasing speakers the biggest regrets, or sometimes bad choices without regret, is looking for a speaker that is too detailed. In the store over 10 minutes it mesmerizes you with the resolution of frequencies you thought you would never hear again.  You take the speakers home and after a month you realize they are ear drills.  High pitched, shrill sounding harpies you can't believe you listened to long enough to make a choice.

The other mistake, which audiophiles life with far too long is buying too big a speaker for the room.  The specmanship of getting 8 more Hertz in the -3dB cutoff is a huge factor in speaker purchases.

What do you think the biggest mistakes are when buying speakers?
My last speaker selection was narrowed down from 5 different makes, I had my own amp hooked up in their demo room and it took me 2 hours to select one, I then explained I would need to try this speaker at home, I had to buy them but on the agreement that if they wasn’t the right speaker for my room I could return for further auditioning, 
As for base and treble, my amp isa Sugden A21SE so no bias adjustment or even balance, and that’s in my opinion the way it should be.
Incidentally the speaker turned into a happy purchase and all is well.

And for all newbies out there, never buy your HiFi from a store that sells washing machines. 
OP:  Always run a long-term home test before buying speakers.
Give yourself plenty of time to climatise yourself to the sound.
You will have years to enjoy your choice not years to regret it.
I am sure you know this.

To all the posts about low ohms and low efficiency, your opinions are nothing but mindless prejudice.  Nothing at all wrong with 4 ohms and 86db.  As has been said, most of the greatest speakers on offer are around those figures.

Forget the specs and judge each speaker on its merits. Use them only as a very broad guide to room matching and amplifier selection. Even then you sometimes get surprising outcomes that shouldn’t work well on paper.
Worst mistake for me is not to hear the speakers in your room with your gear before purchase.
I doubt impedance or sensitivity alone say a lot about the speaker sound. My worst speakers were with 8 ohm and north of 100 dB. Eversince I am around 4 ohm and below 90, increasingly satisfied with every next upgrade. And it is very true they require appropriate amplification. Every thing costs another thing. That’s life.
While I understand some of the arguments around high impedance and high efficiency speakers I also have to ask how often does this happen?  How often does a buyer mismatch his speakers and amps? 

Or is the argument if you aren't buying 16 Ohm/110 dB speakers you made a mistake?