To make things easy for everyone, this will be the approach that we will take to spoilers when discussing anything relating to tv series, movies or anything else where someone may be querying whether discussion would be a spoiler.
Always spoiler any major plot development from something that has not aired yet. If it will ruin the story or experience for someone, please spoiler it. Please used the description of the spoiler to indicate that it is a spoiler of a certain kind. For example: "plot spoiler" or "book spoiler" if you're posting in a tv thread about how the series is different from the book. not everyone in a book thread will have seen the tv show and vice versa. Like so: (note, the spoiler isn't a spoiler in this example. Click away.)
Outside of the thread dedicated to that show / movie / whatever, please use spoilers if you mention something spoilery about that show/etc. For example, don't be in general chatter thread and drop a major spoiler about a movie you just saw. People will avoid the thread about that movie so they don't get spoiled, but they shouldn't have to avoid the entire forum. Also, indicate what is under the spoiler in a similar manner, e.g. "Harry Potter Deathly Hallows Part 2 movie spoiler"
[spoiler="Harry Potter Book Spoiler"]la la la. Harry isn't as hot as Ron.[/spoiler]
Within a thread about a tv show or movie : once it has aired, open the thread at your own peril. If it has aired, people should be free to discuss it if they so wish, including major plot spoilers and developments. If you've not seen it yet, don't go into the thread. If you want to engage in discussion about the series but not the most recent episode, then only read up to the date that the episodes you've not seen yet aired at.