Seven

Don't get adopted by boy-bands.

Don't write about rape which later turns to romance.

Improvise on the original story all you want as long as you can maintain the writer's thread, but you can't suddenly put dragons into, say, the Hunger Games.

Please, please verify countries' currencies when you're writing. Dollars are not used in the UK, and neither are Euros (even though UK is part of the EU.) UK uses pounds.

Don't write a Twilight fanfiction where the vampires don't sparkle.