The characters incorrectly refer to the Headless Horseman as the "Hess-i-an". The correct pronunciation of Hessian, which was a German mercenary from the region of Hesse who aided the British in the Revolutionary War, is "Heh-shen".
The small toy (with string and the picture of cardinal and a cage) that Ichabod plays with in the film is known as a thaumatrope. The thaumatrope was not invented until 1826, several years after the events in the story.
Flashbacks have the 7-year-old Ichabod seeing an Iron Maiden. If Ichabod is at least 25 in 1799, then this scene takes place in 1781 at the latest. This torture device is therefore a minor anachronism, as it was invented (primarily for display in museums) only in 1793.
The costumes worn by Katrina and Lady Van Tassel were from an earlier decade, circa 1785. Colleen Atwood, the costume designer, stated she wanted the costumes to reflect the period 1794. Atwood's rationale was that the residents of Sleepy Hollow were five years behind the latest fashion trends. By 1794, women's dresses were being to reflect the beginning of the Regency period: closer to the body with a higher waist and much less elaborate than those displayed in the film.
In 1799, Ichabod declares "The millennium is upon us. In a few months we will be living in the 19th century." The millennium - the start of a thousand calendar years - was, of course, more than 200 years away. (Also, the 19th century began in 1801, not 1800.)
Ichabod, Katrina, and young Masbath bar the door of the windmill with a shovel and put two sacks on the left side of the door and one on the right. However, when Ichabod says "To the roof; I've got an idea!", there is only one sack on the left side of the door. When the horseman cuts the shovel with his sword, there are again two sacks on the left side of the door.
When Ichabod Crane is playing the bird in the cage game while in the carriage, he removes it from his pocket twice. Once when he first gets in the carriage then again after he looks out of the window at the men removing the woman's body from the back of the wagon.
When Ichabod is brainstorming the connections between various events and writing them in his notebook, the individual words that are shown being written are in a different handwriting than when they appear all together on the page.
(at around 49 mins) When Ichabod cuts up the tree and is splatted with blood, it is on his face and down his neck and white shirt. During the cutting of the tree, the blood splatter on his face changes position with each shot. While digging the hole and following the headless horseman, there is no evidence of blood on his shirt.
When Baltus Van Tassel is explaining to Ichabod about the headless horseman, Baltus is in front of the fireplace, and then when shown again he's over to the right by Magistrate Phillips' chair, and then he is shown again by the fireplace, and then shown again he's back by the chair.
(at around 18 mins) When Ichabod Crane is talking to the men about the Headless Horseman, he sets his cup of coffee on the edge of the table. The preacher drops a Bible on the same table, and the coffee cup is close to the middle of the table. In the next shot, it is again at the edge of the table.
(at around 18 mins) In the flashback showing the death of the horseman he is shown being buried still wearing his combat armor yet when Ichabod digs up his grave in the present (at around 50 mins) all that remains of him is a bare boned skeleton. Having only been buried for a decade or 2 there would not have been enough to completely decompose the armor to nothing since it was made of cured leather and metal.
(at around 1h 35 mins) Toward the finale when the Horseman grabs Katrina to kill her, he is standing within a few feet of the huge massive dead tree and doesn't move a step as he catches the skull that Ichabod tosses at him. In the following scene when his head is re-attached, he is clearly standing at least 10 yards away from the tree.
While at the notary's office, Icabod Crane's hair is neatly combed one moment, then wild the next. Same when he says goodbye to Katrina and then leaves the town. Totally inconsistent during the entire scene.
The servant girl's head was severed by a mortal and not the horseman. Her head and neck lacked the cauterized qualities that the sword delivered. There should have been a quantity of blood due to the ax blow that took her life.
The closing credits thank "The Town of Hertsfordshire". The correct spelling is Hertfordshire (commonly abbreviated to "Herts"), and Hertfordshire is not a town, it is a county. There is a town but it is just called Hertford.
When Ichabod is in the carriage and about to leave town, he is playing with the cardinal/cage optical illusion toy. When the camera faces him the image of the cardinal and cage are right-side up, but when shown from Ichabod's view they still appear right-side up. One of these two shots should show the images upside down.
The capital of New York state moved to Albany in 1797. After that date, any investigator would have come from Albany, not New York City; although as justice at that time was administered locally, it is unlikely that any "state" investigator would be sent, anywhere.
(at around 55 mins) The rifle Brom uses to shoot the Headless Horseman has a scope on it. Scopes were not invented until the 19th century. The first documented telescopic rifle sight was invented between 1835 and 1840.
(at around 5 mins) The film opens showing New York in 1799. Two US flags are shown at the beginning of the film depicting old glory with the familiar 13 stripes (one in New York city and one in Sleepy Hollow). In 1799, the flag contained 15 stars and 15 stripes after Vermont and Kentucky joined the union in 1791 and 92 respectively. The flag was in this configuration from 1795 through 1818 and was the only time in American history when the flag had additional stripes beyond 13. Afterwhich, the flag was reduced to 13 stripes and only stars were added to the flag with each new admission to the union.
When Icabod is riding in the carriage to Sleepy Hollow he pulls the window shade down after the howling of the wolf. When the camera focuses on the carriage from a distance, the shade is no longer down.
In the opening scene, when the little boy is lighting his candle to see the figures on the wall, he sits very close to the candle. So the light from the figures should be visible not only on the wall behind the boy, but also on the boy himself. This does not happen.
(at around 2 mins) At the very start of the film Peter Van Garret in the carriage is looking forward in the direction of travel. He looks out of his left-hand window at a scarecrow. The camera tracks the scarecrow as the carriage goes past. The tracking shot is not correct. It is a tracking shot as filmed from the right hand side of the carriage, but Peter was looking out the left hand window.
The goof items below may give away important plot points.
When Ichabod is investigating Jonathon's Masbath's death, he says to Phillips "and the head?" Phillipse replies "Taken" but in the gap preceding his answer you can see his lips uttering several other words but the sound has been removed.
(at around 1h 35 mins) At the end of the movie, Lady Van Tassel has a scar on her left hand from when she cut herself. After she is knocked out by Ichabod's helper, and then wakes up in the horseman's lap, we see that her left hand bears no scar.
(at around 24 mins) When Ichabod Crane is examining Jonathan Masbath, he leans over and puts on glasses. When he straightens, the telescopic lens is extended. The shot cuts away and then back at which point Crane extends the lens.
(at around 1h 18 mins) In the church, after the killings, Ichabod and Katrina are in a small balcony looking down. Katrina faints. Ichabod notices in close-up, that she has dropped a pink chalk, which lies perpendicular to her fingers. In a longer shot the chalk now lies parallel to her fingers.
Ichabod deduces that the headless body in one of the coffins he rips open cannot be that of Lady Van Tassel because the cut in the palm of the hand was done after death, without, he says, any bleeding. Later, when we see, in flashback, the cut being made, it bleeds profusely.
(at around 1h 35 mins) The skull that Lady Van Tassel carries appears to be a normal human skull, but when the Horseman reattaches it, it is almost doubled in size. But when the skin begins to regrow on it, the skull is its normal size.
(at around 1h 35 mins) At the end of the movie Crane refers to The Bronx, a borough which did not exist yet. However, a river already bore the name of Bronx, so that is what he refers to in the story's context.
There was no reason for Lady Van Tassel to fake her own death, since the movie makes it very clear that in the event of a death the deceased's estate goes to the next of kin. Lady Van Tassel would not be Katrina's next of kin if she (Lady Van Tassel) was already dead.
Logically, Lady Van Tassel should have ordered the Horseman to kill Katrina before killing Baltus. With no living children, Baltus's heir would have automatically been his widow. There is no mention of Katrina having a will so there was no guarantee that her death would make the estate go directly to her step-mother (especially since Lady Van Tassel was already presumed dead.)
When the Headless Horseman is about to kiss Mrs. Van Tassel, you can see his actual teeth behind the filed ones. Christopher Walken's teeth are blacked out and their outline is visible against his tongue during the closeup of his face.