The movie is this: Devil (2010) - IMDb
If you haven’t seen the movie here’s a brief description of the scene: A guard claims that the weird stuff that have happened earlier in the movie are because of the devil. According to the guard, all things go wrong when the Devil is near. To prove it, he butters a slice of bread and flips it in the air. It lands buttered side down.
When I saw the scene I started laughing out loud because the first thing that came to my mind was Discworld. In a Discworld book, death makes a machine that flips buttered slices of bread above small pieces of carpet. If all slices land buttered side down then it is a sign that the Auditors are near.
But what about the scene in the movie? Was it a direct reference to Discworld? Or is this belief widespread? I’ve heard about the buttered bread before, but apart from Discworld, never heard of using it as a test for the presence of evil.