(Approx. :22) The Yogi Bear board game behind the head of Billy's bed changes position.
While Billy and family are traveling in the opening scenes of the film, the radio reports the weather twice as being in the low 20s. Despite this very cold weather, no one in the family is wearing winter coats or gear.
Not only would Billy not be tall enough to impale Denise on the antlers, he would not have the strength to force them through her entire body, and the antlers wouldn't be strong enough to accomplish that task.
Following the murder of Ellie Chapman (Tara Buckman's character), she is shown one more time before the flash-forward to 1974, during which her heart can be seeing beating against her bare chest; even far more visible is the carotid pulse beating against the stage blood on her neck.
When Billy's mother is murdered, the blood appears from the stab wound before the knife even breaks the skin. The slice to her throat then literally opens itself before the killer can slash her throat.
It's highly unlikely that a Catholic school class such as the one at Billy's orphanage, would include anyone but 4th century Saint Nicholas of Myra in their curriculum, certainly not the mythical Saint Nick of contemporary Christmas, let alone do an art project based on Santa Claus, an assignment to write a letter to Santa, hire a Santa to give out presents on Christmas, or a nun to force a child to sit on Santa's lap against their will.
(at around 8 mins) The convenience store that is held up by the man in the Santa suite isn't modified from its 1984 look at all to fit the look of a store from 1971 when this part of the film takes place. This is most noticed when reading the price signs, which advertise prices that are too high for the time period. The poster in the doorway advertises a 6 pack of Pepsi cans for $1.99, which is over three times more then it would have been in 1971, and the sign above the check out counter advertises $1.00 ice bags. Also the Visa and MasterCard logo stickers in the windows have the 1980's logo, MasterCard was "Master Charge" until 1979 and Visa was "BankAmericard" until 1976.
When the children are seen singing "O Come, All Ye Faithful", their mouths are off-sync with the vocals.
Andy confronts Billy on the change in his attitude during the holiday season, since he began working at the store "two months ago", which presumes Billy started working in the summer or fall. The film established earlier that Billy was hired in the spring.