To take an example familiar to most people reading this blog: if you have a child narrator/POV for telling the Biblical story of Noah’s ark, stop when the child is out of the story. Do not then add an authorial note about global warming, animal evolution, or anything else that is modern. If you have a mother narrating the loss of three children in a natural disaster, don’t add an authorial note after the mother’s death that tells how the one remaining daughter became a nun and devoted her life to working with children following natural disasters.

These examples are blatant, but beware of more subtle wrap-ups as well. If you have a wrap-up at all, as opposed to an ending, ask yourself whether it takes the reader out of the story itself, whether it adds anything relevant, whether you can do without it.