I always add on 20% for unexpected overages that often come up.

Screen Resident Thoroughly:Placing the right residents can be vital to your success as a real estate investor and can help you reduce unexpected vacancies.

I'm more conservative in my approach but I'd say you at least need to have enough for the down payment, closing costs, etc. plus a healthy reserve so that you can cover a vacancy of a few months as well as the ability to cover the costs of any unexpected maintenance and/or capital expenditures.

With flips, be careful as you can go over budget and find unexpected problems.

So I gave a 100k buffer to cover any unexpected costs.

I don't budget per se, but I always have a good balance in my account to cover the unexpected. 

Run your numbers and always account for mishaps - making sure you have the funds for the unexpected.

It was hard staying positive especially when unexpected repairs came up, but we stayed the course.

If you put your rehab budget into each of these, would you be able to get it all back if you need to sell unexpectedly?

That part if pretty straightforward, especially if you speak with a good lender, but I'd say most important is to make sure you dont drain your cash at closing--you want a good chunk of cash left over to protect yourself from unexpected costs (I bought my first multi when I was 23 and within a month had a furnace go out).