predecessorObserver.TryGetPath not delivering valid paths

Topics: algorithm, graph
Sep 3, 2014 at 2:38 PM
I am using predecessorObserver.TryGetPath on the categories structure (tree) of the German wikipedia. I want to generate all paths from all category pages to the root category. However I see from my code that way not all categories product a valid path. Please note that I am using an edge cost of "1" for each edge.

Question: Under which circumstances can the above named method be non valid?