Some commonly mistaken terminology in theological statements and discussions:
Tenets are beliefs. Tenants are people who live in a building.
Armenians are people from Armenia. Arminians are people who somehow identify with the free-will theology of Jacobus Arminius.
Calvin is the man's name, so it's Calvinist or Calvinism. No idea who Calvan is or what Calvanism is.
Crucifixion, not crucifiction. That's a not unimportant distinction.
It's the book of Revelation singular, not Revelations plural.
When I started this post, I knew there were a bunch more but now I can't remember them. What more am I missing?