A spoiler is a piece of information that, when released too early, might spoil someone's enjoyment of a game, story, comics, and so on. Anything about plot twists or surprises could be considered a spoiler.
On the Gallowpedia, spoilers are permitted after a game, comic or other material was officially released. Expect to see spoilers if you haven't played the latest series entry or read the latest comic release.
Because there is a general spoiler warning on the main page, spoiler warnings aren't required on individual articles that contain spoilers about recently released materials.