Examples of pitfalls include purchasing in an undesirable area, not understanding the market values (vs. list prices), not knowing the rental values (current and market), inheriting bad tenants, buying a lemon, going rogue on renting and management with no contacts / systems / no legal understanding of PA and Municipal landlord-tenant laws, not having trusted resources and contractors in place, using out of state lenders or shaky financing, throwing lipstick on a pig when renovating a house and wondering why it's ARV isn't increasing, tenants are renting, etc.

If something happens and makes the closing either impossible or undesirable, you lose whatever time and money you've put into that property.Worse, if damage or loss to the property can somehow be traced back to work you or your contractors did, guess who is getting sued. 

You were made aware of undesirable conditions and made immediate attempts to address the problem through a capitol improvement not just a simple repair.

Do not spend time trying to turn an undesirable suspect into a client; spend time FINDING those who need your expertise and product, and are willing to PAY your desired price5.

The "undesirable for incorporation" comment in the wikipedia article is likely to change in the next decade or so IMO.  

When I hear about a situation or action which I find undesirable I think Yuck.

It's those areas that will have lots of cash buyers.That being said, unless you work an area that is really remote, really undesirable or really high end, you are not going have a problem finding cash buyers in this market.

Dont trust anyone or you will end up holding a undesirable property.

Evicted undesirables in one side of the unit and re-rented.    

With respect to the law of building codes, permitting and contractors, I have to be frank--in Ohio, it is nothing more than a hidden tax, a protective tariff, and a way for the municipality to veto any development or renovation that its architectural review board subjectively considers "undesirable."