Occasionally a user may see an error message that he or she cannot perform an operation due to Calendar limits, such as:
Unable to save event. This means you have reached the limit for the number of events you can edit.
There was a problem saving your changes. Please try again in a few minutes. This message will be followed by a more descriptive description on the limits that have been reached.
Why is this happening? Google Calendar puts certain limits in place to protect our users and infrastructure from abusive behavior. When these limits are reached by a user, Google Calendar will go into read-only mode for that user, and all edit actions will fail for a certain period of time. Most users will never hit these limits, as they are well above the activity level of a typical Calendar user.
When a user makes changes to non-primary calendars (such as secondary calendars or a manager’s calendar), all actions are counted under the user's own account and not the limits of the processed calendars. Google Apps administrators who use Google Calendar heavily or anyone using a third-party tool or API to manipulate events therefore have an increased risk of hitting Calendar limits.
If users hit the Google Calendar limits, they should be able to edit their calendars normally again within several hours. Users will still be able to view events in Calendar during that period.
Please see the link below for the full article from Google