What are the painful landlord problems no one is talking about?

[Rene G.]

MY QUESTION: Landlords, what are your most painful problems (that you know how to handle), yet they keep happening?

MY THOUGHT PROCESS: I notice a lot of landlords posting unique one-off problems or situations. It's usually something they never ran into before.

What I never see are veteran landlords posting about their biggest nuisance problems. I guess I want to know what painful problems landlords have that they handle and wish they didn't have to deal with. You know... The things that happen again and again, that you know how to handle, but can't prevent from happening.