I've had the same ~2010 era furnace in my rental property since I bought it and it's required a ~$500 fix in both 2013 and 2016.
Apart from that, just shitty tenants that were my own damn fault.
Oh, and one time a family of two woodchucks and three skunks broke through the floor in the bathroom, but that's just life.
If you buy right, run the numbers conservatively, and screen your tenants properly, you're golden.