Apple calls this the iCal forum, and they say there's no Calendar forum. I'll hope for a congenial reception.
I'm on the latest 10.8 running Calendar. I noticed bychance that I type the letters HM into the search field and I see an event in the drop-down search results that doesn't appear in the search results column. It also doesn't highlight in the calendar itself. A calendar I call Personal. So I exported it, opened the .ics in bbedit, and found the HM event.
SUMMARY:HM 2004 construction
I put a couple of carriage returns in so you can better see the line that says SUMMARY:HM 2004 construction
See the line above that. I don't like the looks of TRANSP:OPAQUE It reads as if to say the event is in the calendar but opaque, hidden from view. There appears to be no pref or setting to make opaque events appear in the Calendar app. There appears to be no information about event opacity on-line.
Mountain Lion might or might not have caused this 'issue', but the ML install would have done whatever it normally does to calendars when installed over 10.6
I'm hoping someone can tell me something the rest of the net hasn't said yet.