Starting with the simple but possibly annoying stuff, (depending on your marketing) be prepared to get lots of calls and questions from buyers and buyer's agents about the property and possible viewing the property.

Even if you have good tenants, annoying nuances can come up in multiplexes.

Could be a lot of things; bad data, bad script, maybe you’re not good on the phone, maybe the investors you’re working for have unrealistic expectations of what a deal is, too much competition... as annoying as cold calling is to me, I know there’s people out there doing it successfully. 

If you’re blast mailing/texting/stalking 2-4 unit property owners that you found from a list you’re not the only person annoying that owner.

That is the biggest most annoying scam that is sold to persons on these units.

I was going to pay it when he called, and my wife was annoyed with a bunch of things wrong with the property and his attitude so I declined.

I had 11 of these and good systems in place but still had 1-2 evictions a year and 3-4 big things that were normal annoyances like plumbing leaks, damages from hurricanes, or some vandalism.

Pro Tip: Get a property manager with in house turn crews to do the rehab for you (should be a light cosmetic rehab) and then manage it for you and you shouldn't have annoying contractor issues.

They know the area and the annoying little rules. 

Out of state investing can be annoying but our dollar stretched further Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?