There's a scene toward the end of the movie, from Wendy's POV, of two men, one in a tuxedo and another dressed up in a bear costume, that is arguably the most bizarre scene in the whole movie, even topping the lady in the bathroom scene. The man in the tuxedo is lying down on the bed, with only his legs visible initially, while the man in the bear suit is kneeling down on the floor at the foot of the bed, his face hovering over the tuxedoed man's crotch. As Wendy looks on in horror and confusion, the costumed man straightens and stares at her, and the tuxedoed man - sensing an audience - sits up and leans into view, and both men stare intently back at her. Now, they aren't naked (although the bear suit is missing its backside, exposing the kneeling man's buttocks), but we can reasonably assume they're doing something sexual. What makes this scene so bizarre is that it's an incredibly short (no more than five seconds), isolated bit of business (we've never seen these men before, nor do they help, hinder, or interact with Wendy in any way) that has no explanation. To understand what's going on, you have to have read the book.
At a point about three quarters of the way through the novel, when "the hotel was running things," as Jack is about to be served his first drink by the Overlook, Danny walks out of the Torrences' apartment within the hotel and attempts to go to Jack and stop the Bad Thing from happening. Blocking his way, however, is a man "dressed in some sort of silvery, spangled costume. A dog costume...." Danny asks to be let by, but the costumed man begins barking and howling and threatening to "eat [Danny] up," starting with his "plump, little cock." The man then makes references to "blowing down" Harry Derwent, and continues to menace Danny until the boy goes back inside the Torrences' living quarters. Later, during one of those time-bending sequences when the hotel brings its past back to life, the mystery man's identity is explained. One of the Overlook's former owners was a man named Horace Derwent, an eccentric Howard Hughes type who poured over three million into restoring the Overlook after WWII, hoping to make it "the Showplace of the World." At one of his lavish masques thrown for the benefit of the rich and famous, Horace played mockingly with one of the guests - Roger - who was dressed up like a dog. During the hotel's "re-enactment" of the party for Jack, a gorgeous woman explains to him that Derwent is bisexual ("AC/DC...although he never goes for repeats on his DC side"), and Roger is a former lover. According to the woman, Horace told Roger "if he came to the masked ball as a doggy, a cute little doggy, he might reconsider (having sex with Roger)." Although no actual sex scene between Roger, the costumed man, and Derwent is described in the book, Kubrick's vision is a logical extension of their relationship.
It's difficult to say why this scene remains in the film, as it's somewhat confounding without all of the setup that King provides in the book. Perhaps its jarring incongruity is reason enough for its inclusion, illustrating as it does Wendy's extreme disorientation at that point in the film. Another explanation is that the background on Derwent may have been scripted and filmed, but excised in the final cut.