YOU are the biggest risk to the process. All the hopes, wishes and bias you come with get in the way of making good decisions. The more you want something to be true, the more you have to kick the tires. So many people try out a bunch of random stuff, find a pattern that has a notional profit and just get blinded by dollar signs. That's when you start to skip over important details and you make all the sort of errors as detailed in the other answers here. Even when you are aware of these risks, they only get lip service while you are too busy grasping at the profit. You need to keep your feet on the ground and make reality based decisions. (Note that when you start to loose money fear takes the place of greed and you continue to make bad decisions)
Remember that in the end your own or your clients wealth will rest on the decisions you make. You will have down days, when you see your wealth falling what will you do? Unless, you have rock solid faith in your process you will start to tinker and go into a death spiral. To have that faith you need to have absolute discipline right from the start.
Luckily, there is a well established approach that has been developed of thousands of years to tackle this problem and it has proved very successful. It's called the Scientific method. At this point people throw up their arms and say that's too hard to understand or I don't have time for this. Well if you aren't prepared to work hard and spend the time, you've already lost. Believe me, I've done this for a long time now and I'm convinced this is the most valuable thing you can learn. It doesn't mean you have to have a PhD, wear a white coat and smoke a pipe.
The scientific method really is your friend here. Wandering away from reality into fantasy is so tempting but will lead to failure. Step away from the P&L! Don't look at the notional dollars instead work out what you know and what you can prove.