Spoilers off for:
- If the copyright has expired, or if it predates the very concept of copyright, it's probably free to spoil. In the vast majority of countries, this roughly coincides with anything Older Than Radio. The cat is out of the bag for all of William Shakespeare's plays.
- Anything from myth (e.g., Oedipus), religion (e.g., The Bible), or even legends (e.g., Robin Hood). If it's Older Than Dirt (or at least Older Than Feudalism), it's definitely OK. That said, occasionally spoiling Biblical events (Jesus dies but comes back to life) can be very funny. Just don't go overusing it.
- If you spoiler-tag a fact, something that happened in Real Life, you have our full and express permission to punch yourself in the face. Unless you do it to be funny, then the person who doesn't realize it's a joke and angrily deletes the tag has the face punching permission. Obviously, this applies to any works based on true stories (unless they're only inspired by the facts and take significant liberties with the historical record).
- Quotes from a work. Either it doesn't need spoilers, or it doesn't need to be quoted; use ellipses (...) if need be.
- If the article is an episode recap, no spoiler font. The nature of the thing is to give detailed information about the episode. A person who wants both all the details and to not see spoilers is not a person we can help. However, do not add spoilers for future episodes, even under tags. Those go on the later episode pages. Foreshadowing is a special case — state only what is doing the foreshadowing, not what is being foreshadowed.
- Fridge, Headscratchers, and the various Moments subpages (Tear Jerker, Moment of Awesome, etc.) are for post-viewing discussions. Spoiler tagging there defeats the purpose of the articles. You shouldn't be going there if you are worried about them.