The unknown/perfect scenario: both tenants happily sign a 30 day move out clause (does this exist?)

I know I've over estimated expenses and vacancy which will hurt projected cash flow but I also assume that there is at least one unknown variable that I haven't accounted for. 

Many places won't do controlled burning exercises anymore due to the unknowns being released during the fire, lead etc. thank the enviro nuts for that.

However, self-insuring unknown risk, such as liability, even with an asset-protection strategy in place, is rarely a good idea.

Do you really know somebody who wants to rent some unknown property from you at some unknown time in the future?

I learned many unknown things just in the five minutes of reading this thread...

If you have a real estate brokerage but you aren't using a big name to back you up (ie: Remax, Century 21, etc)...when you are flipping houses, do you feel that your unknown brokerage name impacts things like appraisals, inspections, permits, etc?

In duplex there isn't much of unknowns but your right you should get financials.

Like who the hell has time to call back an unknown number.

Maybe I'm simple minded, maybe my area is different from others, but not accounting for the unknown (appreciation) there is not enough room in the few analyses I have done to pay a mortgage, cover expenses, and have and reasonable positive cash flow to grow your portfolio without borrowing more money.