What is it that Scotty from Star Trek always says? "You cannot change the laws of physics."
I'd have to agree with the dear Mr. Scott. Many people think that writing fantasy means you can do anything you want. Maybe if you set it outside the known universe (not outside the galaxy but outside the universe), you can.
We write fantasy or science fiction or horror...not delusions...not misinformation. Readers in the know expect a certain amount of fact and reality, even in a fantasy world.
What are some of the worst offenders I've seen?
Rust! What rusts? Rust is, by definition, ferrous oxide. IOW, it's the state that occurs when unbound iron is exposed to oxygen. UNBOUND iron. That's why stainless steel and surgical steel don't typically rust. If it's made right, it's not unbound. It's been forced by heat and blast into a bound state. Any other type of unbonded steel is able to rust, if the bonding is removed, just like iron does.
Silver doesn't rust. Silver does tarnish, but it's a largely non-reactive element. That's why it's used in electronic components on seagoing vessels, where corrosion runs rampant.
Gravity! Gravity can be calculated by the distance between two bodies with mass and the relative mass of each body...as well as other gravitational pulls acting on those two objects. I once read a book (a wall-banger, I admit) where the author had people in Earth's stone age jumping 30 feet in the air. Now, the Earth has gained some mass from falling space junk...just as we've lost mass from objects we've shot into space. We have not gained enough mass in a few dozen millennia to allow this sort of thing.
Authors don't just need to be well-versed in grammar and self-editing. If you're writing about something that has a scientific base to it, you should research it appropriately first. When I wrote TYGERS, I researched brain functionality, seizures, and tigers for the book.
So, what's your opinion? Does this sort of thing drag you out of a story? What are the worst offenders of it you've seen?