|The Homeless Man|
In the film the Exorcist, Father Karras is briefly taken aback to see a homeless man in the subway asking him for money. Later in the film, he hears the words of the man repeated by the possessed Regan, which helps to convince him of the reality of the possession.
The common interpretation is that the homeless man dies off screen, and the echo we hear is merely Pazuzu finding tools in hell to help best torment Father Karras. This is supposedly done again late in the film, when discussing the fate of Karras' mother. However, this forgets the constant sightings we have of the demon through the film, showing his observation is constant, and not limited to any time or place. We don't just get the idea the demon is there. We see it, flashes on the screen, flashes that can move and interact with people. Some suggest that the homeless man was actually a demon, sent as a spy. The line is only repeated to shock Karras, to remind him that he is being watched.
This also resolves the issue of why the demon has access to information about Karras' mother. There isn't any given reason as to why she would be in hell. In this theory, she was being watched during her death for possible ways to use her demise against her son. References to her death and her possible disappointment in her son, along with his guilt, are made at the most opportune moments later.