First, a note on this spoiler policy: I do not care about your general opinion on whether spoilers exist, research done on the matter, spoiler expiry policies from elsewhere, etc. Or rather, I do, but none of that applies here. _Your_ common sense approach does not take priority over these rules. Feel free to discuss what you think of the whole philosophy of spoilers here, but that doesn't mean you can ignore the following*.
1) If the post is about a particular episode then you can post whatever you like about that episode and previous episodes in the comments. Same with movies, books, etc. If I post about the exciting thing on page 617 of Lord of the Rings then talk about you like about any content on pages 1 to 616.
2) If the post is not actually about that. Or is about _more_ than that. Then you must disguise what you are talking about. By "more than that" I include link posts that link to something about the latest episode of a TV show _and_ also link to other things. People may wish to talk about said other things, and I want them to be able to do so, safe in the knowledge that nobody will tell them that the entire cast of Archer are replaced by alien doubles of themselves halfway through season three**.
3) Valid methods of obfuscation include vagueness "I had no idea that that exciting thing was going to happen! I was blown away!",Spoiler Tags, and encryption.
4) To use Spoiler Tags simply wrap your spoiler in <lj-spoiler> - see for more details.
5) In order to encrypt your comment go here, enter your comment and hit the "Cypher" button. This will use the Awesome Powah of ROT13*** to transfer your text into something that looks like this: "Una Fbyb vf Yrvn'f Sngure". You can use exactly the same internet magic to convert it back again. Go on, give it a go. You can also do this using a plugin for Chrome and an addon for Firefox.
That's it. Dead simple. If the post is about X, talk about X all you like. If it's not then be vague. If you can't get your point across vaguely then ROT13 it.
And lo, people never had to have arguments about spoilers ever again****
*God, I wish I didn't have to write sentences like the above. But previous history on this topic (amongst others) has shown that the second you bring this up you have people replying with things like "Well, I don't see how people can possibly not know that in book four of the Zarquon Trilogy Eric gets turned into a goldfish - I got spoiled when I went to Tumblr, so clearly everyone who'se ever heard of it will know that.", apparently not realising that not everyone uses the internet the same way that they do, and that someone announced on my FB today that they were finally going to get into Dr Who and where was a good place to being, as they didn't know much about it. Yes, really.
**I've not seen more than one episode of Archer so far. I really hope that doesn't happen. But if it does, I didn't know about it. So there.
***It converts each letter into a number from 1-26, adds 13 to it, and then converts it back (where Z+1 = A).
****I wish. This used to be standard fucking practice on Usenet, back in ye olden days of the internet, when I were a lad and this were all text as far as the eye could see. I have no idea why I didn't do this years ago.
Original post on Dreamwidth - there are comments there.