Spoiler alert! JAN 27 2014
Spoiler alert: people have been saying "spoiler alert" since the early 80s but it became a popular term only relatively recently. (Also, spoiler alert on that article...it gives up a major third-season Game of Thrones spoiler right at the top.)
"It feels like it's been part of our collective life since the dawn of time," Matt Zoller Seitz, television critic at New York Magazine and editor-in-chief of rogerebert.com, told BuzzFeed, noting that he despises the phrase. "I just hate it," he said. "It sounds like something a character in Clueless would say."
Despite his dislike of the term, Seitz said he wrestles with the concept professionally and vividly remembers his first spoiler experience.
It was May of 1983 -- Seitz was in eighth grade. Return of the Jedi had just come out, and a friend from school who had read the novelization came up to him.
My favorite early use of the phrase is from Usenet in 1982 about Wrath of Khan:
regarding Spock's parting gesture to McCoy, it wouldn't surprize me if that's how they bring him back (if they do); but then, i have a low opinion of ST's script(s). Spock's farewell to Kirk sounded pretty final to me.
Ha! (via @alexismadrigal)