There is a scene in Three Men and a Baby that has apparently become a huge "Hollywood Urban Legend" over the years. I stumbled upon it when watching an episode of TV Land's TV Myths & Legends the other day. Instead of explaining the scene in question (with Ted Danson and Celeste Holm, both of whom have appeared on MPI), I'll just post the links that lay out the whole situation:
Wikipedia
Snopes
Pretty weird, huh? When you watch the scene, it REALLY does look like a little boy peering out from between the curtains! It's really quite creepy, even when viewing it out of context from the rest of the film. It's also very easy to notice in the background. I find it amazing no one noticed it during editing. It's even more amazing that nobody noticed it until the film was released on video.
YouTube clip of the scene in question (:53)
When you look at the closeups, it's pretty obvious that it is indeed the cardboard cutout of Danson's character in a tux, and not a "ghost" or a person standing there. Darn. What I find perplexing is how the cutout in the window changes so much in the one continuous, non-cutaway, scene. When you first see it, it is much farther to the right, and almost entirely obscured by the curtain; you only see the left edge of the cutout, the left arm of the tux. When they walk back past the window again, the cutout is nearly perfectly centered in the window! They say the camera angle and distance has changed, but it sure doesn't appear that way when you look at the two shots closely. It really, really looks like somebody moved the cutout in between the two shots of them passing the window.

How bizarre, how bizarre.