Given that the knowledge about correct parts of a password based on known sources (pi, peace and war, song lyrics etc) drastically reduces the amount of possible solutions. But how would an attacker figure out the first part of such a password? What comes to mind are timing attacks http://en.wikipedia.org/wiki/Timing_attack What other possibilities did I miss?

EDIT: I get that having a long streak of my pass in a dictionary would reduce overall security but it's still unclear how a partial match in the dictionary would be detected.