First, you should back up your contacts and groups:

1. Open Contacts.
2. Back up your contacts by choosing File > Export and selecting Contacts Archive. Specify an export location and then click Save. The resulting archive file will contain a backup of all of your contacts information that you can restore later into Contacts (if necessary) by double-clicking it in the Finder.

Next, proceed with troubleshooting:

1. Remove any duplicate or unnecessary blank entries. Select an account from the Groups sidebar. If you don’t see the sidebar, choose View > Groups. Choose Card > “Look for Duplicates.” Click Merge. Contacts creates a single card for each contact using the name and picture of the card that was listed first. It may be necessary to repeat this procedure until duplicate entries no longer appear.
2. Choose Contacts > Preferences, then click the vCard tab. Ensure the 3.0 option is selected for vCard Format and that both the “Export notes in vCards” and “Export photos in vCards” options are selected.  
3. Create a new folder on the desktop.
4. Select all of the contacts in the Name category in Contacts by choosing one of them, and then choosing Edit > Select All.
5. Drag all these contacts into the new folder on the desktop by clicking and dragging one of the highlighted names; this will result in one vCard generated with all of your contacts.
6. If you have contact groups in your Contacts, select and drag each group to the new folder on the desktop (a vCard is generated for each group).
7. Disable syncing using the appropriate method for your operating system.
   OS X Lion and later
   
   1. In the Finder, choose Go > Utilities.
   2. Open Terminal and execute this command:
      defaults -currentHost write com.apple.syncservices SyncingDisabled YES
   Mac OS X v10.6 Snow Leopard and Mac OS X v 10.5 Leopard
   
   1. Open the iSync application and navigate to iSync Preferences.
   2. Disable the option to "Enable syncing on this computer."
8. Delete all of the contacts and groups in Contacts.
9. Quit Contacts.
10. Hold the Option key while clicking the Go menu in the Finder in order to select the Library folder (if you are using OS X 10.6 or earlier, choose Go > Home in the Finder, then open the Library folder), and then open the Application Support folder. 
11. Locate the "Address Book" folder and move it to the Trash.
    
12. Hold the Option key while clicking the Go menu in the Finder in order to select the Library folder (if you are using OS X 10.6 or earlier, choose Go > Home in the Finder, then open the Library folder).
13. Open the Preferences folder in the Library folder.
14. Move the following files to the Trash:
    
    • com.apple.AddressBook.plist
    • com.apple.AddressBook.abd.plist
       
15. Hold the Option key while clicking the Go menu in the Finder in order to select the Library folder (if you are using OS X 10.6 or earlier, choose Go > Home in the Finder, then open the Library folder).
16. Drag the files located inside the "Caches" folder to the Trash (do not move the "Caches" folder itself, only its contents).
17. Choose Go > Go To Folder from the Finder, then go to this location:
    /Library/Caches
18. Drag the files located inside the "Caches" folder to the Trash (do not move the "Caches" folder itself, only its contents).
19. Log out of your user account (choose Apple > Log Out in the Finder).
20. Log in to your user account.
21. Reset the SyncServices folder using the appropriate method for your operating system.
    OS X Lion and later
    
    1. In the Terminal window, type or paste the following command on a single line:
       /System/Library/Frameworks/SyncServices.framework/Versions/A/Resources/resetsync.pl full
    Mac OS X v10.6 Snow Leopard and Mac OS X v10.5 Leopard
    
    1. Open the iSync application and navigate to iSync Preferences.
    2. Then choose iSync > Preferences > Advanced > Reset Sync History.
22. Re-enable syncing using the appropriate method for your operating system.
    OS X Lion and later
    
    1. In the Finder, choose Go > Utilities.
    2. Open Terminal and execute this command:
       defaults -currentHost delete com.apple.syncservices SyncingDisabled
    Mac OS X v10.6 Snow Leopard and Mac OS X v10.5 Leopard
    
    1. Open the iSync application and navigate to iSync Preferences.
    2. Enable the option to "Enable syncing on this computer."
23. Open Contacts.
24. Drag the vCard from the folder you created on your desktop to the Contacts window to re-import them. To re-create contact groups, create the groups first in Contacts and then drag the appropriate vCards to each group.
25. Attempt to sync normally using iTunes for your iPhone, iPad, iPod, and iPod touch, or use iSync for other mobile devices.
26. If the contacts still do not sync, try starting your computer in Safe Mode by pressing and holding down the Shift key after hearing the chime on startup. It may take several minutes to start your computer in Safe Mode. After starting in Safe Mode, restart your computer and then attempt to sync normally.  
27. If your contacts don't sync, consider contacting AppleCare for support, or see this article for more information about removing the Sync Services folder. However, removing this folder is strongly discouraged, and should only be performed when the syncing Mac has been started in Safe Mode.
28. Once your contacts and groups sync correctly, create a new backup of your contacts, using the procedure outlined in step 1. This will preserve any of the changes you made that fixed your sync issue.

First, you should back up your calendars: Back up your calendars by choosing File > Export > Calendar Archive in OS X Mountain Lion or File > Export > iCal Archive in OS X Lion or Mac OS X v10.6. In Mac OS X v10.5, you can back up your iCal data by selecting File > Back up iCal.

Next, proceed with troubleshooting:

1. Quit Calendar.
2. Disable syncing using the appropriate method for your operating system.
   OS X Lion and later
   
   1. Open Terminal and execute this command:
      defaults -currentHost write com.apple.syncservices SyncingDisabled YES
   Mac OS X v10.6 Snow Leopard and Mac OS X v10.5 Leopard
   
   1. Open the iSync application and navigate to iSync Preferences.
   2. Disable the option to "Enable syncing on this computer."

3. Hold the Option key while clicking the Go menu in the Finder in order to select the Library folder (if you are using OS X 10.6 or earlier, choose Go > Home in the Finder, then open the Library folder).

4. Open the Calendars folder. 
5. In the Finder, delete the Calendar Cache files, including any that also display a -1 (or -2, -3, and so forth) directly after their filename. 
6. Choose Go > Go To Folder in the Finder, then go to this location: 
   /Library/Caches
7. Drag the files located inside the "Caches" folder to the Trash (do not move the "Caches" folder itself, only its contents).
8. Reset the SyncServices folder using the appropriate method for your operating system.
   OS X Lion and later
   
   1. Choose Go > Utilities in the Finder.
   2. Open Terminal and type or paste the following command on a single line:
      /System/Library/Frameworks/SyncServices.framework/Versions/A/Resources/resetsync.pl full
   Mac OS X v10.6 Snow Leopard and Mac OS X v10.5 Leopard
   
   1. Open the iSync application and navigate to iSync Preferences.
   2. Choose iSync > Preferences > Advanced > Reset Sync History.
9. Re-enable syncing using the appropriate method for your operating system.
   OS X Lion and later
   
   1. Open Terminal and execute this command:
      defaults -currentHost delete com.apple.syncservices SyncingDisabled
   Mac OS X v10.6 Snow Leopard and Mac OS X v10.5 Leopard
   
   1. Open the iSync application and navigate to iSync Preferences.
   2. Enable the option to "Enable syncing on this computer."
10. Open Calendar.
11. Attempt to sync normally using iTunes for your iPhone, iPad, iPod, or iPod Touch, or use iSync for other mobile devices. If the sync does not work, attempt to sync only one calendar at a time, noting which calendars do not sync.

If the issue persists and is isolated to specific calendars, follow the expandable steps below (organized by operating system) to try to locate event titles that could be problematic during sync troubleshooting.

Check your event titles for the following:

• Check to see if any calendar events have blank or empty titles. If so, rename these events so they have a title or delete the event.
• Check for calendar entries containing any events using high ASCII or double-byte (Unicode) characters in the event title (such as Japanese, Chinese, Korean or other extended ASCII characters). Try removing these characters or substituting them with non-extended ASCII characters.
• Check for and remove any duplicate calendar entries.
• Consider removing some old events if you have a very large number of them.

Using OS X Mountain Lion and later

1. Open Calendar.
2. Type a period in the Search field located in the top right corner.
3. All of your calendar events will appear in the search results. Depending on how many events you have, it may take a few minutes for all of them to display.

Using Mac OS X v10.5 to OS X Lion 

Using Mac OS X v10.4 or earlier

If you are unable to resolve your issue by locating and renaming potentially problematic events titles, continue troubleshooting with these steps:

1. Disable syncing using the appropriate method for your operating system.
   OS X Lion and later
   
   1. Open Terminal and execute this command:
      defaults -currentHost write com.apple.syncservices SyncingDisabled YES
   Mac OS X v10.6 Snow Leopard and Mac OS X v 10.5 Leopard
   
   1. Open the iSync application and navigate to iSync Preferences.
   2. Deselect the option to "Enable syncing on this computer."
2. Remove all the calendars in iCal.
3. Reset the SyncServices folder using the appropriate method for your operating system.
   OS X Lion and later 
   
   1. In the Finder, choose Go > Utilities.
   2. Open Terminal and type or paste the following command on a single line:
      /System/Library/Frameworks/SyncServices.framework/Versions/A/Resources/resetsync.pl full
   Mac OS X v10.6 Snow Leopard and Mac OS X v10.5 Leopard
   
   1. Open the iSync application and navigate to iSync Preferences.
   2. Choose iSync > Preferences > Advanced > Reset Sync History.
4. Re-enable syncing using the appropriate method for your operating system.
   OS X Lion and later
   
   1. Open Terminal and execute this command:
      defaults -currentHost delete com.apple.syncservices SyncingDisabled
   Mac OS X v10.6 Snow Leopard and Mac OS X v10.5 Leopard
   
   1. Open the iSync application and navigate to iSync Preferences.
   2. Enable the option to "Enable syncing on this computer."
5. Using the backup you created previously, import one calendar into Calendar by choosing Import from the File menu. Verify that you can sync this calendar.
6. Continue to import and sync one calendar at a time until all of them sync successfully or you discover one that does not sync.
7. If your calendars still don't sync, consider contacting AppleCare for support, or see this article for more information about removing the Sync Services folder. Removing the Sync Services folder is strongly discouraged, and should only be done when your computer has been started in Safe Mode.
8. Once your calendars sync correctly, create a new backup using the procedure outlined above. This will preserve any of the changes that you made that fixed your sync issue.