This happens when you use one of the Trace tools (Double Trace, Quadruple Trace, Satin Line Trace or Motif Line Trace) to follow an outline in the picture and then Trace finds a part of the outline that is very thick. This can be caused by excessively broad outlines or by selecting too much of the picture with a high Color Tolerance value. In this case, Trace does not know where to trace the outline as the problem part of the outline is more like a fill area. Use a lower Color Tolerance value, or change the picture in a picture editor to make the outline thinner or break up the outline so that the wide parts are isolated from the rest of the outline.