There's a difference between "Top 10 legal mistakes" and "Top 10 mistakes" made by founders. Something can be a legal mistake (as in, causes legal problems down the line) and still be a brilliant strategic move. As CEO, it's your job to weigh the legal hot water against the business benefit and decide what you need to do.
Disclosing your idea without an NDA means you may lose exclusive rights to that idea. This is often a prudent course if the alternative is that you fail to gain traction, go out of business, and have to get a job, where you lose all rights to your ideas. Oftentimes, the goal of a startup is to live to fight another day, and strategically it's a good move to take a loss on one particular idea so that you can continue to generate ideas in the future.
Disclosing your idea without an NDA means you may lose exclusive rights to that idea. This is often a prudent course if the alternative is that you fail to gain traction, go out of business, and have to get a job, where you lose all rights to your ideas. Oftentimes, the goal of a startup is to live to fight another day, and strategically it's a good move to take a loss on one particular idea so that you can continue to generate ideas in the future.