10 Ways to Jerk Your Reader Out of Your Fantasy World

Use a modern word in a medieval fantasy.

Modern feminism transported back in time. (Lots of kinds of feminism in lots of time periods.)

Modern politics in the future.

Pontificating characters when action should be moving forward.

Sexiness when we should be planning for battle.

Revealing your ignorance about how gravity and other basic principles of physics work.

No other languages in your fantasy (sf) world. (Ditto other cultures, myths, religions)

Megapowerful beings who intervene to protect humans just when things turn for the worse.

Changing your rules of magic when it is convenient to the plot.

Characters who believe in everything that modern people believe in, and who sneer knowingly at the common misconceptions of the past.