Did you know that Titanic is a movie about time travel? The less eagle-eyed cinemagoer might think the film mainly concerns the sinking of a giant ship. They would be wrong.

Leonardo DiCaprio's character Jack Dawson gives several hints that he may be from the future. He regales Kate Winslet's Rose with tales of ice fishing on Lake Wissota in Wisconsin, a manmade lake which was filled in 1918 -- six years after the Titanic went to its watery grave in 1912. He also mentions walking on Santa Monica pleasure pier... which only opened in 1916. Even his gear is ahead of its time: when Jack rushes on board the Titanic at the beginning of the movie, he is toting a 1939 Swedish Army rucksack.

The more plausible explanation for these anomalies is that director James Cameron and his team simply made some mistakes. Shock, horror -- hand those Oscars back, sir! If you think that is a bit of an overreaction then congratulations: you are not a film faux pas freak.

While most movie fans prefer to concentrate on the big picture -- the acting, the plot, the atmosphere -- committed anoraks make it their mission to focus on minor details.

These disgruntled nitpickers' idea of a good night in is to compile lists of continuity mistakes, prop inconsistencies, plot holes and historical errors in our favourite TV and movie shows.

The BBC has been the focus of their ire in recent weeks. The broadcaster's adaptation of classic spy novel The 39 Steps annoyed certain viewers when it featured a number of anachronisms.

John Buchan's novel on which the TV drama, and a 1935 Hitchcock movie, were based is set on the eve of the First World War. In the Beeb's version though, the hero lives in a 1920s-style Art Deco building. There is a car chase between a 1924 Morris Oxford and a 1927 Wolseley. Three of the 24 viewers who made complaints to the BBC were concerned that an aircraft used in a scene over the Scottish moors was an SE5a, not yet in use in 1914.

Ben Stephenson, the BBC's head of drama commissioning, insisted that period drama does not have to be strictly accurate.

"We take the feedback on board but, for me, the purpose of drama is to entertain, not to be slavish about detail. I think that absolute dedication to perfect detail is something for a documentary and not something for a drama," he said.

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Authenticity over accuracy is the argument most commonly used by programme and film-makers when they are 'caught out' by blooper fans. The BBC also found itself having to defend several out-of-time props featured in its Life on Mars cop show, set in 1973.

One character is seen wearing a .. clunky gold-/analogue Casio watch, a type which did not appear until the mid-1980s. CCTV cameras and satellite dishes are clearly visible in what is meant to be 1970s London. A Mark 4 Ford Cortina is used by one of the lead detectives -- that model of Cortina only went into production in 1976.

Ironically, the show features a detective who finds himself thrown back to the 1970s after being involved in an accident in the present day. When he wakes up in 1973, he is lying on a building site beneath a large hoarding advertising the coming construction of the Mancunian Way. One small problem: that motorway was completed and opened in 1967. The detective, it seems, is not the only time-travelling entity on the show.

Historical details were also overlooked in the BBC-HBO co-production Rome, and in The Tudors, which is shown on both the BBC and TV3. Rome was heavily criticised for its highly sexualised aristocracy.

Historians contend that ladies of high class in imperialistic Rome simply would not have been so sexually open. The timeline of major historical events was also manipulated to suit the storytelling.

The Tudors was derided by his-torians from the second it pre-sented Jonathan Rhys Meyers as the rampant Henry VIII. Golden Globe-nominated he might be, but the Irish actor is too young for the period of Henry's life depicted in the series. When Anne Boleyn was in her 20s, for example, Henry would have been in his 40s.

The Tudors also uses Elizabethan costumes and Victorian carriages with springs that hadn't been invented in Henry's time.

More amusing are the little modern touches that have popped up on the set of The Tudors: radiators in the king's bedroom, tarmacadamed driveways, concrete bollards and a modern-day metallic measuring tape used to measure Johnny Rhys-Meyer's inside leg made it a bit of a Carry On Henry.

Most of us won't have seen -- or possibly care about -- any of these bloopers. While you or I use the pause button on a DVD only to go and make a cup of tea, those with a little more time on their hands (too much, some might say) use it to freeze every frame on the lookout for continuity mistakes.

There are even whole websites dedicated to categorising and listing the offending gaffes: www.moviemistakes.com, www. nitpickers.com, www.movie-bloopers.com, www.slipups.com and the brilliantly titled www.nerdnirvana.org are favourite posting sites.

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Even geekier are the groups who look for specific slip-ups. One IT nerd has become so hot under the hard drive about computer mess-ups in movies, that he has listed 10 of "the most egregious" mistakes. His bête noire is film heroes who can walk up to any computer in the world and figure out its operating programme in five seconds.

"That is just ridiculous," he opines.

A group of military jet enthusiasts are similarly exercised about the identification of aircraft in films, which surely makes them a new generation of plane-spotters. They are perfectly scandalised about the scene in Independence Day where Will Smith and his pals set off to save the world.

"Nobody does a pre-flight check of their planes," remarks one horrified chap, "They just high-five each other and hop in."

As for the eggheads who point out James Bond's regular trespasses against engineering, mathematical and scientific principles... Well, one imagines that credibility with physics professors is not of huge concern to a character that can get his hands on a rocket-firing cigarette.

There is some method to the list-mania of the blunder-watchers. Blockbusters can rake in millions selling a version of history or time to audiences worldwide: the least they can do is try to make it feel authentic and keep their story straight. It must be noted that some continuity errors that appear on the final cut of a film are often spotted by the script supervisor but have been let through on purpose. Good takes from an actor are often more important than continuity.

A director will always choose to edit in a particularly brilliant bit of performance, even if there is a chair in the corner of the frame appearing and disappearing between shots.

In defence of the nerds, it is always fun to point out the car driving past in the background of a Biblical epic movie.

As one of the movie mistakes sites contends: "To err is human, to point divine."