I would say a common error that people make is underestimating the cost to fix or repair something.

If this is becoming common in certain areas, do you think it might deter a significant percentage of potential guests?

This is incredibly common in Philadelphia's market if you don't invest in the correct neighborhoods.  

This is fairly common in college towns so I'd recommend verifying that before going off on a buying spree.

First, I would recommend calling around to apartments and other PM companies in your area to find out what is common. 

Most postal carriers have enough common sense to recognize which box is for unit A and which is for unit B.

It’s a very common misconception.

The most common is to move in family or a rehab extensive enough that unit cannot be occupied.  

As I understand it 100% deduction on expenses that are soley related to the business or the rented unit, 50% deduction on expenses that relate to the general property as a whole ( mortgage interest, insurance premiums, common area repair expenses).

Roots are very common in cast iron as it slips in between the joints.