I've had two occasions where the reference they listed told me not to rent to them because they were violent, late payers, or too needy.Not doing a criminal, credit, background check, which might have revealed criminal or credit problemsBeing slow to evict (mentioned by others)Believing their sob storiesAccepting partial paymentsFailing to document every conversation or interactionNot walking through the property to make sure they're taking care of it (I now walk through every 9 months so I can see problems in different seasons)We've found crack pipes and other paraphernalia after they moved outWe've discovered piles and piles of trash (some bagged, some not) in closets, basements, sheds when trash cans and pickup service are included in their rent - all they had to do was carry it out, at no cost to themWe've Found smokers, pets, extra peopleOverall, I've had great success as a property investor.