Root Problem: We've got some weekly events - sometimes "Recurring", sometimes not. It isn't unusual for someone to get confused and RSVP for the meeting from two weeks ago, instead of the upcoming meeting.
It's not hard to wind up on the wrong event, of course: the wrong object code, an old link, a notification email, or anything of the sort can lead someone to an older event. They see the title (which rarely changes), don't check the little date below it, and bombs away.
Possible Solution: It would be fantastic if the Calendar had a warning somewhere if you are viewing/editing/acting on an event that is in the past.