Spoiler (netabare)
Overview
A spoiler (netabare, the Japanese word for revealing a story's secrets) refers to revealing the key developments, ending, or surprise elements of a story to someone who hasn't experienced that work yet.
Sensitivity to spoilers differs from person to person. Some people can enjoy a work even after learning the ending, while others lose interest if even a little information gets through. Trouble over spoilers partly comes from these individual differences. Information that seems trivial to you can be a spoiler that ruins the fun for someone else.
The spoiler problem in question boxes
In question boxes, spoilers become a problem in two ways. The first is when a sender sends a question that contains a spoiler. A question like "What did you think about Y dying in the finale of X?" becomes a spoiler for a person answering who hasn't watched the work yet. The sender may send the question without knowing what the person answering has watched.
The second is when the person answering publishes an answer that contains a spoiler. Because answers in a question box are made public, an answer containing a spoiler may ruin the fun for readers who haven't watched the work yet. Even an answer you think you're giving to a single sender becomes a message to an unspecified number of people.
Dealing with spoilers
Preventing spoilers in a question box depends largely on the care of the person answering. When a question containing a spoiler arrives, state clearly at the start of your answer, "The following contains spoilers for X." This one sentence gives people who don't want to read it a cue to stop scrolling.
A more careful approach is to keep things vague in your question box for answers that would contain spoilers, with something like "I can't go into detail because it would be a spoiler...," and talk about the details somewhere else (such as a community where spoilers are okay). Keeping in mind that a question box is a public place, and trying to give answers that consider their effect on the people who read them, also helps prevent trouble over spoilers.