The biggest fiction (constrained by budget obviously) of shows like Star Trek is that most of the intelligent creatures we might possibly meet will look almost exactly like us. I don't think even the people coming up with Star Wars aliens have the imagination to get it right. They still base it on what we are limited to thinking up as humans and our own likely narrow understanding of what is life and what is intelligence.

The second-biggest fiction is that it would be possible for us to coexist on one planet's surface considering our needs when it came to gravity, atmospheric pressure and basic atmospheric composition would be very unlikely to be the same.

But that would narrow the scope of a lot of sci-fi, so I let it go.