Until they stop paying rent and trash the place and squat and sue you for any imperfection such as accidental loss of heat in the apartment, while simultaneously not letting the boiler repairman in the house.
I for one could probably only afford to be a landlord of a 250-400k property here in NY, not a 700k one!
Strong lease agreements, property management, and vetting the right tenants will avoid 99% of these issues... but you can only do so much to prevent these issues from arising. I account for 3-5% vacancy every month as part of expenses and don't touch that money for this very reason.
It's not that common, but it's a possible outcome, and a contract won't help even a little bit.