Google Calendar's support page mentions some peculiar behaviors that happen in rare situations. Google's solutions are somewhere between strange and hilarious.
Why am I receiving a corrupted screen on Google Calendar?
If you’re receiving a corrupted screen while accessing Google Calendar, please clear your browser's cache, sign out of Google Calendar, close your browser, and restart your browser. In some cases, it may be necessary to clear your cache more than once.
I keep receiving repeated SMS messages from Google Calendar. Help!
Occasionally, if the communication between your mobile device and your mobile providers doesn't go smoothly, you may receive duplicate SMS message notifications. When this occurs, just turn your mobile device off and on a couple of times, and see if that makes a difference.
If that doesn't fix the problem, please contact your mobile provider and ask them to reset your SMS message settings.
Why do I receive a 500 error when importing events?
You may receive a 500 error when importing events from iCal or CSV files that contain a large amount of data. If you receive this error, wait for five to ten minutes with your browser window open and then close your browser. Next, open your browser and sign into your Google Calendar to see if your events were imported.
If your events have not been imported, try importing events from a smaller range of time.