Spoiler alert: In “Titanic,” the boat sinks.

OK, that’s silly — just like the “Peanuts” strip where Lucy intentionally spoils “Citizen Kane” for Linus by blurting out the object to which the deathbed utterance “Rosebud” refers.

Nevertheless, determining what constitutes a “spoiler,” the unwanted tidbit that ruins one’s viewing experience, remains very much in the eye of the beholder — such as the patron of the Showtime series “Dexter,” who complained that my second-season review gave away too much about the months-old first season, which she happened to be watching on DVD.

Among the side effects of time-shifting, TiVo-ing and DVD-viewing culture is the complete breakdown of etiquette governing what should and shouldn’t be said about movies or TV series, where the potential spoilers are refreshed on a weekly basis.

Chiding a recent New Yorker piece that revealed plot points about the upcoming season of HBO’s “The Wire,” Rachel Sklar noted on Huffington Post, “There are no codified written standards” defining spoiler protocol, but at least a modicum of consideration would be prudent.”

Equally telling, however, may be the very urge to spoil — a strategy initiated by overly enthusiastic Internet geeks eager to lord their knowledge over pretty much anybody, which has subsequently been adopted by traditional media outlets yearning to boost web traffic. Given the competitiveness of the modern age for any crumb of “exclusive” information, the result has steadily lowered the bar on what qualifies as a scoop.

Granted, this is partially due to the shows themselves becoming richer and more complicated. There wasn’t much that could be done to “spoil” an episode of “Bonanza” or “All in the Family” in the same way one can mar “Lost” or “The Office.” The fact remains, though, that “Heroes” twists are of dubious news value in the bigger scheme of things, beyond the fuzzy hope of finding a way to translate clicks into bucks.

This discussion of spoilers, by the way, comes from someone who, in the past, had key revelations in “The Sixth Sense” and “The Empire Strikes Back” blown by chatty nerds, so the practice engenders no sympathy here.

Moreover, any thinking journalist should recognize just how unwise it is to risk alienating consumers. Given the depressed state of the newspaper industry, the last thing any writer should want to do is dissuade people from reading a paper or offer them less incentive to buy one.

At the same time, the web has enhanced immediacy and capacity, allowing outlets such as the Los Angeles Times to ape TV-crazed sites and chronicle every wrinkle in popular shows, as old media struggles to keep apace of the cultural zeitgeist.

In these instances, writers must be cautious, warning those who might have recorded a show, and planned to watch a few days after its broadcast. Nor is this an insignificant audience subset as DVR penetration rises, with the latest seven-day playback data boosting tune-in for several primetime series by a million viewers or more, led by “Grey’s Anatomy” (2.1 million).

Yet as the “Dexter” exchange demonstrates — or another email from a woman enjoying the 2006 PBS miniseries “Bleak House” on DVD — establishing a reasonable statute of limitations on spoilers is more confounding than it sounds, from reporting on TV casting changes that might denote characters having been killed off to mentioning details of the Harry Potter books when some people see the movies only.

As a Variety colleague pointed out, these questions are hardly new. In 1992, when Jaye Davidson was nominated as best supporting actor in “The Crying Game,” merely listing him in that category triggered some protests, inasmuch as that exposed the movie’s secret for anybody that hadn’t yet seen it.

At a minimum, the changing dynamics demand applying more thought than just flashing “Spoiler alert!” to absolve publications of responsibility, counting upon that phrase to stop readers in their tracks.

But for how long? And in the final analysis, if by some miracle you still don’t know what “Rosebud” means and you’re reading about “Citizen Kane,” well, whose fault is that?