Most people live in a "home" which to me is quite different than when they want to move.  Putting a house on the market for sale is important to remember since you want to attract a potential purchaser.  Hence, merchandising that product becomes a necessity.  

Failure to address first impression & curb appeal followed by their pricing practices. Put on a buyer's hat and see clearly what is going on followed by professional or common-sense feedback & input instead of invoking the golden rule i.e., I have the gold, and I rule process to succeed. 

Not dealing with the obvious deferred maintenance issues

Failure to reduce clutter or oversize furniture making the rooms look smaller than they are.  

They go overboard in updates spending way too much money figuring they'll get an even higher price.

GREAT question!
I'd say it's a huge mistake to leave ANY 'remnants of seller' in the house.
Sentimental items, family pictures, get those out of there!

The other is to not repair any known issues prior to market. Quit lazily saying 'as is' when you KNOW stuff is wrong and YOU wouldn't buy it like that!

Trying to sell it themselves.

I am with Fred Griffin Florida Real Estate on this.  If the right choice of agent is made everything that needs to be addressed will be addressed to avoid or prevent any "mistakes" provided sellers listen and follow recommendations. 

Choose the wrong Listing Agent

I have had two sellers over the years, that had beautiful homes.   Decorated beautifully, but too much.   Big pieces, and when on the market, less of the decor is better.

Not getting a pre-listing inspection

Too much stuff in the house, basement and garage.

Not get their home clean and "staged" for buyers.

Hello KAREN SANCHEZ  clutter, the house not clean enough.

By Far it's having too much clutter and furniture in the house.

Good Q! I think that sellers forget their house is a commodity that they are trying to sell to someone else. Deferred maintenance will always come back & bite them in the arse.

Too much clutter - no one cares about your spoon collection. Get it off the wall.

Price, doesn't want to replace carpet or paint because they don't know what color new buyer wants, and odors.

Interesting questions and answers.  I agree with all.  clutter and maintenance are two issues which seem to ring loud and clear.

Many.... I'll just bring one of them:

I often hear: I do not want to deal with any repairs, let them ask for a credit and do it themselves the way THEY like. It is a big mistake. Fixing small things make a big difference, especially if you expect top $$$.  Most buyers want a house to be ''move in ready''. When they see small cracks, scratched walls, broken cabinets, etc. ~ they just quickly  leave or send in a low offer. 

not prepping their house for sale.

     They can't do it when it comes time to sign on the purchase and sale agreement. Seller's remorse. It happens to sellers not just buyers. And some property owners think they can, but others won't leave until... well you know when KAREN SANCHEZ !

     Mentally prepare your seller. Ask them to wait a week and see do they still want to sell, where are you going to move to, what if that location is not available, etc. Some sellers are not prepped for the move but go deer in the headlights scared and retreat back into their home since 1967.

not listening to their agent

painting bright colors

They paint the home "personal" colors or the fix/repair/modify things that don't need to be done, or they do the wrong things like putting in new flooring but leaving the 40 year old kitchen untouched.  Waste of a flooring job, since a new kitchen means a new floor has to be run.  I've seen a few others over the years, like installing a pool cover instead of fixing up the inside of the house.