Revealing mistake: When the Headless Horseman is about to kiss Catrina's stepmother, 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. (01:37:20)
Revealing mistake: When the whole town's in the church and Katrina's father gets the pole through his stomach, if you watch while he gets dragged through the graveyard, he hits a headstone and it moves back. (01:18:10)
Revealing mistake: When Brom (disguised as the horseman) throws the jack-o-lantern at Ichabod, there is a shot from Ichabod's perspective showing the jack-o-lantern flying toward him. It jiggles from side to side, revealing it was travelling on a wire.
Revealing mistake: In the scene where Ichabod and Young Masbeth enter the witch's cave, the witch tackles Ichabod to the ground. The creepy face of the witch is obviously a mask because you can see the black cloth where the eyes are if you pay attention closely.
Answer: As revealed by Lady Van Tassel to Katrina during the film's climax, the midwife Mrs. Killian was abreast of the secrets regarding the affairs of Peter Van Garrett with the widow Winship, as well as their unborn child. Mrs. Killian revealed this secret to Lady Van Tassel right in front of Mr. Killian, which signed both of their death warrants. However, Lady Van Tassel most likely commanded the Headless Horseman to kill the Killians (as opposed to just saying Mr. & Mrs. Killian), to which the Horseman would instinctively murder their child too. It may have been an oversight on the part of Lady Van Tassel, as the child would undoubtedly be ignorant of affairs and the intricacies of legal matters regarding wills, but then again, she probably didn't care anyway.
Phaneron