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Unfortunately, it doesn't occur to too many people until they're in deep trouble. They just begin subsidizing a business venture to tide it over a rough spot and too often before they're done, they're done in...
The _real_ lesson is don't let personal finances intermingle w/ the business.
If, of course, you're running as a sole proprietorship you're hosed anyway; as it's all the same as far as liability. The LLC helps some but the only real protection is the incorporation route and then you have to be religious about keeping stuff that belongs in the business in the business...