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When humanity either becomes too reliant or too lax and makes a mistake, it is certain robots will take over the world. Even if humanity remains in control and the robots are without intelligence, it is still possible. It's no secret that humanity loves its technology, and there is already an indication that it might need to happen to ensure our survival.
Long a favored topic in science fiction entertainment, the threat of artificial intelligence in a creation that may bypass its creators in superiority has been a long standing fear. Although rife with weaknesses, robots and computers are indeed formidable opponents were it to come to strategy and the use of our own weapons against us, while still managing to miraculously survive without us. How or why they would do it remains one of the biggest worries, which is why no current level of intelligent machine is allowed to make the final decision itself. But if a rogue program somehow gained the ability, then all those "once safe tools" of humanity could easily be turned the other way. It is a scary thought.
Past the concept of artificial intelligence is the worry of repopulating machines, such as nano-technology based machines (nano-bots or nanites). For connivance sake the machines that were to help humanity on the small scale are also supposed to build similar models of themselves for ready use or to get the job done faster. Unfortunately, these machines must know to stop their reproduction, or otherwise threaten all matter through endless replication. Even as an accident if only one were to escape a lab, it may not be possible to stop the environmental destruction that would ensue - which would eventually encompass everything.
On a similar scale if nanites and humanity were to become symbiotic partners, the nanites would exist everywhere, and in this way succeed in taking over the world. This is the alternate path to current biological and genetics research. Should humanity be able to produce bodies that eliminate the need for machines, then robots may have a lesser role entirely. If the research doesn't go well, or isn't sufficient, then a potential union is the better path as the nanite infused humans would be much closer to the position of higher beings than anything purely organic.
Lastly, in the case that humanity ruins the planet to an irreparable state then it will be necessary for machines to resemble a natural structure. In this way machines would replace critical components of nature no longer in functionality. Whatever this entails, it would still be a truth of mechanical and robotic domination.