So do movie spoiler alerts ever expire?

We all agree that in writing about new films, spoilers should be kept to a minimum, right? But what about movies that are decades old — classics, whose stories are embedded in popular culture? Is there a statute of limitations on when it is not okay to openly discuss a film's secrets?

Gary Thompson, film critic for the Philadelphia Inquirer, recently wrote about the anniversary of M. Night Shyamalan's The Sixth Sense, and he issued a "spoiler alert for a 20-year-old movie" before referencing the film's now-iconic twist. An angry reader chewed him out for revealing the surprise.

"There is a subset of readers that will never forgive spoilers, even if there are spoiler alerts," Thompson said. "Unless a work of art has achieved massive and long-standing cultural penetration, you run the risk of alienating readers. You can probably say that Hamlet dies. Or that Sonny dies on the causeway. Beyond that, you are on perilous ground."

Don't be too sure about Hamlet. Matt Zoller Seitz, senior television critic for New York magazine and editor-at-large for RogerEbert.com, said a reader recently went after him on Twitter for revealing the ending to that more than 400-year-old play.

To tell or not to tell? That is the question for thoughtful film critics, who at heart are film lovers, respectful of the film-making process. Even the hint that there is a plot twist can affect the movie-watching experience.

"I will mute certain words on Twitter — Tarantino, Hollywood — so I don't see any spoilers," said Alex Kasemir, 25, as he waited in line recently to see Quentin Tarantino's Upon a Time ... in Hollywood.

"I don't even watch trailers," said his friend Michele Demars, also 25, "which a lot of times give too much about the movie away."

Ann Hornaday, chief film critic at the Washington Post, said critics should adhere to their version of a doctor's Hippocratic oath: "Do no harm."

"As critics, we do tend to forget that movies are an emotional experience for people, even if it's just having fun," she said. "I almost take it as a personal and professional challenge to try to convey as much as I can that will be useful about the film without actually giving away what happens."

Bruce Willis and Haley Joel Osment star in The Sixth Sense.

That's a reasonable goal with new movies. But what about old films? And very old films?

"There is no such thing as an old film," insisted director and former critic Peter Bogdanovich, who is adamant on the subject of spoilers. " If I haven't seen a movie, I don't care if it was made now or in 1920. If it works, it works."

A friend of Orson Welles, Bogdanovich was highly displeased with a 1973 Peanuts cartoon that spoiled the classic twist at the end of Citizen Kane.

Linus is watching the 1941 film on television when Lucy blithely tells all. Linus screams in agony.

Bogdanovich has a historic vantage on the no-spoilers movement. Attending a press screening of Alfred Hitchcock's 1960 shocker, Psycho, he said, "We all thought it was a picture about a woman who steals some money." Then Hitchcock pulled the rug out from the audience with a horrific, out-of-left-field plot twist. "I've never heard this before or since," Bogdanovich continued. "The audience — a thousand paying customers — shrieked through the entire sequence."

The spoiler alert became part of the film's ad campaign: "After you see the picture, please don't give away the ending. It's the only one we have." (The marketing campaign for Jerry Lewis' 1963 Jekyll-and-Hyde comedy The Nutty Professor took this approach one step further. Its posters urged, "Please do not reveal the middle of this picture!")

"We should be even more careful [about spoiling] older films," said Bogdanovich, who is devoted to the cause of getting more people to watch them. He recently released a series of his five-minute Peter Bogdanovich Recommends clips, originally produced for CBS News in the 1980s, to YouTube, to tout the contributions of classic cinema. "There are so many films people should know about but don't," he said.

Brad Pitt, Leonardo DiCaprio, and Al Pacino in Quentin Tarantino's film Once Upon a Time in Hollywood. Photo / AP

But Seitz thinks spoiler sensitivity has begun to go too far. "We are a culture of babies when it comes to this issue," he said. "If I'm going to meet everybody else halfway and try to be sensitive about not revealing the plots of things, I think there has to be courtesy in the other direction."

While he would never with malice aforethought reveal a film's plot twist, he argued that "everyone else is not obligated to tiptoe around your personal viewing habits. I don't know what movies you've seen. I don't know how much of a sense of film history you have. Things are going to get spoiled. Deal with it."

The internet, meanwhile, has made things difficult for even the most spoiler-averse filmgoers.

"I haven't seen Quentin Tarantino's new picture, but I know the ending," lamented director John McNaughton, whose Wild Things is one of the most jaw-dropping and twisty films of the past 20 years. "Even if the writer includes the words 'spoiler alert,' I'm not going to stop reading mid-sentence."

There are countless posts on the web with titles like 36 Movie Plot Twists That'll Honestly Mess You Up a Bit. You can even watch those plot twists on YouTube. There is actually a website devoted to movie spoilers.

"Today the movie is almost irrelevant," Hornaday said, "because everyone is discussing everything whether the movie has come out or not. They're critiquing the trailer. There is a miasma of commentary. There are generations who, we hope, are going to be discovering classic films. Why should they get the short end of the stick?"

Encyclopedic film buff and director Alan Arkush did his best to avoid the Once Upon a Time ... in Hollywood spoilers and was deeply satisfied. "I walked into that theatre having no idea how it was going to end. It was great."