You pick up an historical novel anticipating being swept away to another time and place. You're immediately drawn into the story.
Historical events line up with the proper dates.
Fashions and hairstyles have apparently been researched with care. Even the characters' table manners seem authentic. Yet somehow the story doesn't quite ring true. The fictional world of the novel you're reading suddenly fades from view, and you're no longer a part of it, merely a detached observer. Most of us have probably had this experience.