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?

A nice fat margin can get very thin, very quickly if anything goes sideways. 

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

Its' not the rule but certainly common that filth grows with long term tenants.

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

I've been experiencing lenders tighten there underwriting, and a 1.3X DSCR has been somewhat common in my experience.

This is more common that you might think.They do not take into consideration the costs borne by the investor associated with the transaction such as: closing costs, carrying costs, finance costs, exit costs, etc.They are trying to hit a homerun on every deal and not leaving enough "meat on the bone" for the next person. 

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.