Seldon's plan was based on psychohistory. The way it is described in the foundation series is the prediction of future based on the actions of very large groups of people or societies. However, in the book Foundation - we see that key turning points in the book are actually due to individual decisions - not that of a large group of people.
So - how is the Seldon Plan said to have been thrown off completely by the appearance of the Mule ( I think this is mentioned in Foundation's Edge) when the events of Foundation itself should have thrown psychohistorical predictions off track?