I actually used to service these things under warranty. And we had our fair share of people comming in saying its saying paper jam but there is no paper in it, or that it dog ears the paper or crumples any paper that goes through it. about 90% of the time there was something in there wether is be a small piece of paper, a pencil, pen, bead, glass and even a set of keys.
There are small pieces of plastic placed in a number of different positions all through the paper path (usually called flags) it usually under one of these that you will find something