There is a class of cinemagoer for whom the sharpest pleasure is the chance to spot double yellow lines in the roads of Sherlock Holmes' London. By Kevin Jackson

Fox-hunting may or may not be the sport of kings, but gaffe-hunting is definitely the sport of anoraks. There are many cinemagoers for whom the greatest pleasure offered by the world of film is the chance to spot a boom falling into frame, a pistol that fires one shot more than its chamber can hold, a glass that mysteriously refills itself between shots. The American movie magazine Premiere has a regular column in which consenting blooper-watchers can gleefully share their sightings.

And yet there is one area of gaffe that will have all but the purest of us getting in touch with our inner anorak: the anachronism. Not, that is, the howlingly silly kind of anachronism you find in One Million Years BC, in which, flying recklessly in the face of fossil evidence, palaeolithic man and woman share their turf with dinosaurs, but the jarring little detail that sneaks into a period production despite the film-makers' best efforts to get it right. Trifles like the double yellow line that appears in the roads of Sherlock Holmes's London in Murder by Decree; or the tug steamer that putters behind the longboats in The Vikings; or the wristwatches all too visibly sported by actors in Ben Hur (which also boasts a car in one wide shot) and El Cid; or - a particularly charming case, this - the potatoes on which courtiers feast in The Private Life of Henry VIII.

What makes all such gaffes so amusing - even touching, for those easily touched by human fallibility - is that they would not have taken very much effort to correct. A slightly more attentive cameraman, assistant director or props manager could have smoothed them all away and left a seamless illusion of authenticity, provided you don't mind the sound of Vikings speaking 20th-century American. (And why should you? Shakespeare's Romans and Athenians speak in English pentameters.)

Notably less amusing are those historical inaccuracies that are themselves the product of history's darker passages. Military historians and aviation buffs will point out that the Messerschmitts flown in Battle of Britain are all too obviously the post-war kind made for the Spanish air force, which had a radically different outline from those used by the Luftwaffe since they were fitted with Rolls-Royce Merlin engines - by a piquant twist, the same engines that put Spitfires in the air. Similarly, the panzer tanks in Battle of the Bulge look oddly similar to the American tanks they are pitted against.

In both cases, the howler was inevitable. Short of rebuilding Nazi tanks and planes from scratch, there was nothing much the film-makers could have done, and until computer-generated images become less expensive, just about every form of period technology you see on screen is likely to be out of time.

It's odds-on, for example, that whenever you see a steam train - such as the 1954 model that chuffs its incongruous way through the remake of The Thirty-Nine Steps - it will be from the wrong period. Indeed, even more recent forms of locomotion can't always be trusted. There's a shot in the Who's film Quadrophenia, set in the Mod days of the Sixties, in which a high-speed InterCity train zooms past Wormwood Scrubs.

Moments like that may cause a chuckle for the crowd, but they don't generate the satisfying inner glow of superiority that comes of spotting a gaffe from one's own specialist area. There are lots of anachronisms in Jan Bucquoy's autobiographical ramble The Sexual Life of the Belgians, some blatant, some recondite. One sequence, which takes place in the Fifties, shows the hero's family listening to a BBC World Service broadcast about IRA gunmen being held under the Prevention of Terrorism Act (not so jarring, presumably, when the film is shown in Brussels). For movie nerds, however, Bucquoy's truly delightful goof is a quick flash of the poster from Jean- Luc Godard's Numero Deux, a film made a full decade after the episode the image graces.

Cultural ephemera such as movie posters, pop songs and book covers are at once the most fertile and the most legitimate hunting ground for the pedant: legitimate because they are always carefully placed so as to give the precise spirit of the age, fertile because some imp of the perverse seems to dog every such effort at pinning down the Zeitgeist in an object.

In The Doors, Oliver Stone's camera dotes on the books Jim Morrison scarfed down in his film-student days, one of which is a volume of Nietzsche not published until the mid-Seventies. (But then, Oliver Stone's ideas of accuracy are not those of other men.)

Good snooty fun, all of it; but do anachronisms really matter? Are they really damaging? On the whole, no. After all, only lunatics would read or watch Macbeth in the hope of clarifying their ideas about the dynastic intricacies of Scottish royalty. And yet the cinema's implicit claim and obligation to represent the real, however exotic that reality may be, is so deeply established that it's easy to see why both directors and viewers should have come to make a fetish of historical accuracy.

It is more than simple prissiness for reviewers to gripe that the attitudes, the demeanour, above all the dialogue of certain period films is hopelessly anachronistic - for instance, the Nineties psychobabble spouted in the Twenties of Bullets Over Broadway.

And conversely, it is reasonable for directors working in non-realistic ways to shove wilful anachronisms in their viewers' faces - the typewriters in Derek Jarman's Caravaggio, the helicopters in Alex Cox's Walker, the punks in Alan Rudolph's The Moderns - not just for the perverse hell of it, but to force pedants to stop sniffing around for howlers and to start watching the film.