Bitcoin's 0-confirmation transactions have three major flaws:

1. They are too slow for certain types of microtransactions such as clicking the mouse to access some Internet resource.
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Note many people may think that instant microtransactions are irrelevant, but when that ends up being the future economy of the Knowledge Age and displaces the retail and tangible economy by several orders-of-magnitude, then the dinosaur (Model T) will be Bitcoin.