If you need to get going quickly to ride the wave of a fad before it fizzles, then fast, bare_bones planning may be all you've got time to execute. This works best when you've already got the infrastructure in place, perhaps from previous projects or an established business, and you can simply shift energy and resources to the new idea.
Don't Make Assumptions About Customers _ To be an entrepreneur does require plenty of self_confidence, sometimes almost a bloody_minded determination to make your business work. But this confidence spilling over into thinking that you know what 'the market' wants can be dangerous, without checking that it's true. You need to do your research that the market does ultimately want what you will be offering, whatever products or services you will be selling.