Learn about the iSync 2 (included with Mac OS X 10.4 Tiger) feature "Reset Sync History."
If you're troubleshooting iSync 2, you might wish to reset the computer's sync history by selecting the Reset Sync History option (in iSync preferences). After this, the computer act as though it is being synced for the first time, and you may be asked to merge or replace information on this computer with information from other devices or computers.
Note: This reset will also enable syncing of all calendars and contacts on all devices, even if that was previously disabled. If after performing a Reset Sync History you don't want to sync calendars and contacts with certain devices, reconfigure each device's settings in iSync.