For instance: Isaac Asimov's Three Laws of Robotics aren't based on any practical science, they exist as a plot device for setting off mystery stories with robots and morality plays about hubris. And the reason robots have positronic brains is that positrons were recently discovered at the time, and it sounded cool. Yet people will swear Asimov is one of the hardest SF authors around.

Sometimes you might get a SF author who's an expert in a particular field or has a specific hyperfixation, and that one aspect of their stories might be grounded somewhat in plausibility, but everything else turns out to be complete nonsense.