In my talks with most business owners the main mistake that most make is thinking that it is a light switch - off to on in one flip. Especially with online businesses they assume that once they launch the website that the traffic and money will be immediate.
Unfortunately it is not that simple and many overextend themselves with the plan that they will recoup their investment in just a few days after their website is live.
I would advise anyone opening an online business to develop a plan for the launch involving SEO work, and most importantly realistic expectations. Don't quit your day job to launch your business, launch it and devote your nights and weekends until you get the ball rolling.