Two landlords in particular didn't know about things like escrow account and interest laws for security deposits, contact information posting for vacant landlords, who's responsible for things like snow removal and smoke detector maintenance. I left those units in good condition (thankfully for them!) but nearly sued twice and, in one case, received triple damages on my security deposit.
One landlord said "To keep costs low, tenants generally organize snow shoveling" I said "Oh, sorry, I didn't realize we were breaking the law here."
There are some states with very few tenant rights where landlords can get away with a lot. California and Massachusetts are not those states. It's a very serious part time job that requires a lot more than just owning a property.