Where to begin with things to avoid there are soo many.
As a Designer for over 25 years I have seen the gamut.
#1 Hiring the least expensive Installer
This is usually where the issues start and often goes along with not hiring a flooring professional and using a handyman or contractor.
A professional flooring person will know what to look for and understand the differences in products and be able to advise as to the best course of Installation.
Even with all my years as a Designer I still differ to my Installers when they recommend a particular installation.
#2 Choosing the least-expensive products.
As with all things, there are major differences in types of flooring. These can range from the wearable surface thickness in wood flooring, to knock-offs in ceramic tile. We have specified a particular ceramic tile from our local showroom, only to have the Contractor or client purchase "the same thing" from a discount store. Once the product was delivered it was clear it was not the same in everything from the size of the pattern, to the quality of the tile.
#3 Improper floor prep.
If you are changing out old wood floors to new ones or maybe replacing tile with wood, the floor has to be clean of everything, sometimes even sanded down to remove prior cement or residue. You may also need to nail the subfloor to avoid soft areas that will flex and creak.
Even brand new homes may need floor prep if the initial concrete is not level.
#4 Understand your thickness.
It's a big trend today to put LVT/P (Luxury Vinyl Tile/Plank) over your existing flooring.
That's not something we like to do. It sounds easy however you have to take into consideration your doors, and base as everything in the house will now need to be raised or shaved to accommodate a higher floor. It's worth asking a good installer if you should refinish your current floors first.
Additionally, we have had clients choose new cement tiles for their existing bathroom and do not realize the tile will be significantly higher than the adjoining wood floor. The transitions from room to room need to be considered.
#5 Not understanding how long things take.
Just because the wood or tile is in stock and you are ready to go, does not mean it's a one-day job. Always ask the installer, Removal of an old floor may take one day, but if you need to level or skim a floor that will add a day.
When setting tile there is "Cure" time and tile needs to set before you can grout or even walk on the floor.
Wood floors often need to Acclimate and sit in the space for a few days to let the moisture content equal out to prevent swelling and warping or drying out if Installed to quickly.
Refinishing an existing floor can take 3-4 days, but you might not be able to set furniture on it, use ladders to paint, or even put a drop cloth for up to a week.
People often forget to ask the question as to how long will the job take, and when can we move things back. It's not always the same time period.
That's my top 5 but there are many more.