Unforeseen ways to fail

[Joseph Sanders]

So my business partner and I are getting ready to do our first deal; flipping a house. After talking with him today, I had a question that I thought would be great for the biggerpockets forum. Before I ask the question I want to give some background information:

I know that you want to have the purchase price and repairs to be 70% of the resale value. I have a contractor that I trust with giving me a good price as well as knowing what needs to be repaired and how much it will cost. I have a real estate agent that has experience working with investors and I trust that he will give me a fairly accurate ARV and sell price. So with being armed with all of this, my question is; If we do our due diligence and buy right, what are some unforeseen ways that could cause us to lose money and potentially fail on our first deal?

Thanks for the input!