The ghost put that cardboard cutout behind the window. The ghost controlled everybody involved in that movie. The ghost lives there still.

Thank you! Well said!

For starters, it wasn't left behind the window, it was a reflection of it in the window. Secondly, the "ghost boy" story involved the boy haunting the home where they shot the film (apparently, he died there long before filming began). The home in the movie is not a real home, it is an interior set piece.
Lastly, there are many small mistakes like this left in films, even in modern films. Look hard enough and you'll spot the odd boom mic, costume slip up etc. Two major films where this happened was Star Wars: Empire Stikes Back (the snow monster in the original release is clearly wearing a baby link t-shirt on the shot where his arm is cut off). The second, LOTR: Fellowship of the ring. (When Sam & Frodo are walking through the Shire together, a car can be seen driving in the far ditance, although this is said to be removed since the Extended DVD's were released. The second mistake on this film, when Frodo falls in the snow and drops the ring for a moment, the end of his fake hobbit foot can be seen).
Why, thank you, Thing!