Melancholia was a dark and slightly depressing movie but with some interesting special effects. I watched it on a recommendation from Hal. However, they got the science wrong on a couple of basic accounts and I think I know why.
If a terrestrial planet (dirt and rock) came that close to the Earth, there would be two major effects. First, the tidal force on the Earth would be extreme. The Moon's diameter is more like the size of the United States and look at it's tidal force. Such an approaching big planet as the fictional Melancholia would basically put the Earth's surface through a food processor before any collision ever occurred. I'm not sure if the tidal force would be enough to completely destroy the Earth, but anything and anyone on or near the Earth wouldn't know the difference.
Tidal force is caused by the difference in gravitational force between the closest and farthest parts of one body caused by another. This effect of the Moon on the Earth causes it's ocean surface to bulge toward and away from the Moon. The Earth's crust also does the same but to a lesser and non-catastrophic extent.
Second, the two bodies would move about their mutual center of gravity. The Earth would be perturbed completely out of it's orbit by the encounter in some crazy way. It could still involve a single close encounter, again with both planets being destructively disturbed by the tidal forces of each on the other, followed by a later, final impact, but there's no telling what their mutual trajectory would look like. What is nearly certain is that the Earth would not continue on it's nearly-circular orbital path.
So surely they had a science/astronomical advisor for the movie. Why such fundamental mistakes? I propose that when they consulted their expert, they were thinking of a small asteroid collision with a body more the size of a large metropolis or a state. A small body wouldn't perturb the Earth from it's orbit and wouldn't have much tidal effect. However, the impact would be catastrophic.
Later, because they had brilliant ideas about the look and effect of a giant planet approach and final impact, they changed the threatening body but they didn't go back and consult the expert. They just kept the path from the previous discussions.