“ Have the PCs rping Orcs or Goblins or some other race that normally exists only as things to cut up with swords.”
“ A cult's theology is binding on the god involved: it is possible for that god to believe (and <b>see</b>, just like Winston Smith in 1984) that other gods are mythical, or jumped-up demons, or failing older spirits, even while those gods actually exist and thrive and act in the cosmos, even while working against him.”
“ A small, primitive village has yet to discover fire, which is a good thing because there are highly flammable (but not otherwise dangerous) gasses in the air. The heroes, blissful in their ignorance, don't know that and think it a wonderful idea to show the locals open flame.”