What is the most expensive landlord mistake you've made?

[Luis Martinez]

I’ve been thinking about this recently because it seems like most landlords (myself included) learn things the hard way.

For me, one bad tenant ended up costing thousands between missed rent and repairs. Curious what others here have experienced, what’s the most expensive mistake you’ve made as a landlord?

Also, I came across the National Landlord Convention happening June 4–6, 2026 in Columbus, Ohio, and it looks like they focus heavily on learning from real landlord mistakes and systems. Has anyone here attended before?

Would love to hear if it’s actually worth it.

[Patricia Steiner]

This:  according to the American Apartment Owners' Association the #1 reason why landlords fail is:

Failure to manage to the lease. 

The lease rules.  

I would also offer this: Never forget that you're running a business.  Tenants will attempt to make their problems yours - they need to pay the rent late, they have to smoke - have to, more.  And, if you make one exception, they will test to see if you'll do it again and again and again.  Don't start.