Eleven

If you're writing Doctor Who fan fiction and using a variety of companions, don't mix up your chronology.

When writing 5SOS fan fiction (I don't know too much about them, sorry) keep in mind all the rules for writing One Direction fan fiction. Hell, for writing any fanfic about a band, 1D rules apply.

While writing Game of Thrones fanfiction (as rare as it might be) keep in mind that the story is extraordinary and complex and the intricacies will drive you insane.

Not every beautiful girl has waist-long hair.

People cannot be muscular but unathletic at the same time.