Actors 'fluff' their lines when they forget what they're supposed to say or use the wrong words.

The cause of Regency Romance Raves 'n Rants was the insertion, BY AUTHORS, of distinctly American English words into the thoughts and mouths of the Regency period English aristos.

Thus Fluffs refers to clearly obvious errors in the texts of Regency Period 'historical' romances (those I come across!)