When Jack's mother comes to visit Mary, you can see in the background what appears to be a little boy standing in front of a window. There is a rumor that this is the ghost of a little boy who died in the apartment in which the film was shot. This rumor is false, as the interiors were all shot on a sound stage in a movie studio. The "ghost" is actually a cardboard cut-out of Jack wearing a tuxedo. This prop appears later in the film, when Mary's mother comes to collect her. Some argue that "it's smaller" or "it's dressed differently", but close examination reveals that it really is the cardboard prop.
In the same scene when Jacks mother comes to visit Mary, when she walks past the sofa to the right there is a prop of a rifle standing on end right behind the sofa as they walk across the room. When they walk back across the room to the left, the cardboard cut out is in the window.