My dad was a middle class landlord. He bought the duplex we lived in and rented out the other side and eventually moved into a better place and rented both sides. People move out. People become delinquent and have to be evicted. If you can't handle unexpected periods without rental income then you've blundered into the business of owning rental property. He had all kinds of problems and periods without rent coming in. He always found a way to keep in the black with the bank.