During a recent Drupal 6 to 7 upgrade i stated receiving some bewildering error meesages along the lines of;

Notice: unserialize() [function.unserialize]: Error at offset X of Y

Long story short, it's some antiquated variables stored in the 'Variable' table of the database. The quickest way to identify the offender is to install Variable Check module, enable it, go to /admin/reports/variablecheck and delete the sucka that be causin yo grief!