Ignoring the competition is your worst idea ever. Just because you ignore it, it does not mean that it will disappear. In fact, not knowing what your opponents are doing will most likely kill your business. For example, if your competition sells the same product for 75% of your price, how many products are you going to sell?
Not paying attention to the market saturation is often fatal for a business, especially if you run a local one. It is said that there is room for everyone out there, but this is wrong. If you open a new barbershop in a small neighbourhood that already has three prestigious barbershops, you better include some chips, a beer and half prices too. Otherwise, your business will die.
Not understanding what you are actually trying to sell is going to ruin you in the long run. Helena Rubinstein is the first self made lady millionaire. She has never really sold face creams. Instead, she sold beauty. She liked to say that women are not ugly, but just lazy sometimes. Therefore, it makes no difference if you sell a product or a service. Craft your unique selling quotes accordingly.
No one can tell whether or not a business idea is good without having a plan. Therefore, not doing a business plan is usually the biggest mistake. No entrepreneur or business professional can make predictions without it. In other words, this is a main reason for proper planning.
The individual who invented the idea that you must do what you love should be hit with a hammer. It sounds violent, but this saying is responsible for countless failed businesses. In theory, it looks good. In reality, most people love things that they are curious about and not good at. From this point of view, you should do what you are good at and not what you love.
Not conducting any market research is a serious problem for a lot of enthusiasts. A lot of people start businesses just because they might work, only to go heartbroken when they collapse. Test a service or a product before investing in it. Otherwise, your success is only a matter of luck.