Spoiler Alert

1. When talking about a movie to an audience that has not yet seen the movie, this phrase is used by a speaker to warn that they are about to reveal plot details that may be surprising or unexpected. The thought is that revealing certain plot details will detract from the natural viewing experience, so the warning serves as an opportunity for people to stop listening/reading.

1a. The same idea, but in the context of other media such as books, television shows, etc.

2. Often this phrase is used jokingly following, or immediately proceeding information that is felt to be obvious.

1. A spoiler alert is where someone gives away the main plot or turning point in a movie, book, show, etc.

2. Something that someone uses to really piss people off.

a phrase uttered by someone before they ruin a movie/book/video game and by extention the next 3 days for you

Someone is going to die.

43 Impact, 25 Range, 46 Stability, 15 rounds in the magazine, 360 RPM, 56 Reload, and 68 Handling

A solid gun... if you're not planning on surviving your next firefight.

Usu. a term to describe when crucial elements of a movie are about to be revealed (i.e. the ending, character deaths, a twist, etc.)

1. A phrase, sentence, or really any form of communication that completely ruins that show, movie, or book series by giving away key plot points or twists. interjection
2. Ruining everything for everyone.

When you accidentally spoil something for someone and then attempt to make it up by yelling out "Spoiler alert!"
It will nonetheless piss someone off because either a) you're just rubbing it in that you spoiled the ending to an amazing movie/book/show, or b) they weren't really paying attention before and would have missed the spoiler, but because you had called attention to it, now they are aware they have been spoiled.