Not paying a down payment. Always pay a percentage down payment. If things go as planned, you will get your money back with the sale.
<~~~ You can't get a loan today with no money down. It just doesn't exist anymore. Lenders are requiring investors to put at least 20% down in most cases. That's a big reason there isn't near as much house flipping going on now.
Not having a plan. Decide what you are going to do before you make the purchase. You want the property to be in your hands for a short amount of time. If you go in with a plan ready, you will make a quicker sale.
<~~~ Time lines are key. Sticking with them is hard to do sometimes but you need to get that sucker fixed and on the market QUICK!
Not shopping around for contractors. Get several bids from contractors for the work that you cannot do yourself. Also, get everything in writing including a timeframe for the work to be completed and penalties if it is not.
<~~ This is one of the biggest reason I don't do flips anymore... bad contractors.
I believe you should alway have your investment inspected as well.
<~~ We always made our offers contingent on inspection. Once it was accepted we would have a pro home inspector come in and check everything out. We get the water and gas turned on for a day to make sure everything works. Great idea!