My worst mistake was deviating from our plan. We tried to price our rents to get the best bang for the buck, so to speak, balancing maximum rent but keeping it at such a level to have several people apply.
This was several years ago, but the tenant had high debt and several accounts with bad credit, but cash flow was solid. We took a chance and let emotions get involved to help them out a bit. The calm demeanor evident in the application process soon vanished. I soon got text essays complaining about all sorts of things. And when questioned on treatment of the property or complaints of other tenants, the accusations and text essays flew. They always paid their rent, but it was an ordeal. I was happy to grant their request to terminate the lease early.
Unless you're running a charity, stick to the plan and don't let emotions cloud your decisions.
That was a long, slow ordeal, unlike the time a tenant flashed a butcher knife at me...but that's another story.