What's the most popular startup advice is plainly wrong?

Bad advice: "Your idea? Someone's already done that."
If you think your idea brings something unique and of value to the market, it doesn't matter that's its been tried before. Just do it better and never be satisfied that it's as good as it can get.

To say that there is a lot of crap advice given for business start-ups, maybe right, but a little to focussed on the negative. To compare it to speed daating for example. 10 men or women in front of you and hopefully after initial assessment you will have decided 1 is a good match for you, but it doesn't mean the remaining 9 will never find a partner and success in a relationship. Hopefully you will know more than the basics of the industry you are entering and know what you want to achieve. Deal with the basics re finance, administration, personnel and get the right people doing their job with clear measurable objectives and let them get on with it. You only need a business plan in the traditional form to help raise finance or encourage strategic partnerships. My personal discussion with you would surround the financial sustainability of your first few years engaged in the project and try to ensure that you don't find yourself 'broke' and wondering why.

This quite interesting question, I love to subscribe and hearing what Anphabers will talk about this.

How about "make carefully business plan"? Most of my friends who's already success at stat-up saying that they are not plan at all and tell me that the best plan is not plan at all

Worse is "Start with a product", a good product blablabla... You need to "Start to find clients" clients need. You need to be able to finding 1, 10, 100, 1000 clients and keep them satisfied, whatever the product really! Who can you find every day that God makes who will be happy to buy from you, whatever you are capable of selling them. A start-up and an entrepreneur need to be able to find clients for the rest of the life of the company.