While minor drips or small stains may be considered normal wear and tear, a large and excessive oil stain that requires significant cleaning effort or solvent treatment goes beyond that.

Call the cops saying they are running a drug operation and they get arrested.

This would qualify the property for long-term capital gains treatment. also, if you command decent rents, it’s going to help you flip that property at a higher price point with solid tenants.I also agree with the sentiment that if you decide to flip it and pay tax that’s a good problem to have because it means you made money on your flip and that’s the goal!

Drug dealers start camping in the backyard (kept using extension cords and water from my units). 

But what if he has a drug history?

Drugs, Violence, and boardups all around. 

Most mold is on the surface and can easily be removed with a bleach solution or one of the many mold treatment products available.

People in recovery need a place to stay free of drugs and alcohol.

Personally, I've seen tenants granted a continued occupancy/renewal of their lease for the following:(1) threatening other tenants/neighbors with physical harm(2) dealing drugs out of the apartment with children residing in the same building(3) discharging a firearm on the premises(4) refusing entry to the landlord/his agent for necessary repairsAnd I am sure there are many other examples of bad behavior that the property owner is powerless to react to.

Drug trafficking, sex trafficking, prostitution, people hiding from the law, etc.