The biggest thing we had issues on early on was improperly comping, or estimating of rehabs. A couple of our first deals ended up falling through because we didnt take into account (or understand) these things:
1. Renovations on bigger houses must be run differently than renovations for 1500sqft or less properties. Dont use the same $20-$30-$40 x sqft that you would normally use for a 2500sqft house. Watch Jerry Nortons video "How to comp in under 60 seconds" and it will give you a great start.
2. When comping make sure to take into account the price per sqft sale price. An ARV house that is 1300sqft CANNOT be directly compared to a house that is only 1000 sqft. Even though it is very similar, price per sqft is what many cash buyers are looking at.
3. Location, location, location. If a house is in a bad neighborhood where all the comps are selling for very low prices, take into account that buyers are going to be paying significantly less because of the low upside. Not to mean that it cannot be sold, but with houses that need work in bad neighborhoods, you need to make sure its a good price!
4. Try to stay within the neighborhood if possible when looking at comps. Our mentor even advised us to look at older comps (over 12months) before ever leaving the subdivison. If you do have to leave the neighborhood to find comparable sales make sure the houses are at least similar. Keep in mind no sales in the neighborhood in the last year is typically a bad sign.
Having a deal fall through is never easy, especially when you're starting out. Don't be scared, but make sure you have carefully checked all the things listed above when estimating your buy price. Best of luck to you!