Error popups from Apple Calendar application
Close the Calendar Program
Remove the two caches (Or squirrel them away someplace, just in case.)


~/Library/Calendars/Calendar Cache

~/Library/Caches/com.apple.iCal


Open Calendar and wait for it to synchronize.


This takes care of most problems that are do to some sort of caching issue.

[For Advanced users]
If that doesn't work, you can disconnect from the networked accounts, export backups of you local calendars (make sure you have good solid backups of everything!), then move ~/Library/Calendars out of the way. Start Calendar, restore your local calendars, and re-connect your network calendars. Once everything synchronizes, your calendars should be clean.

If you continue to have issues, you may have to be patient and wait for Google to fix an issue with their servers.