I use Calendar on Mountain Lion to manage my Exchange calendar. In order to see other people's calendars, I had to mark them as delegates so they appear down the left side of the app. Fairly often (suspect it's when I disconnect and reconnect to the network and exchange), some of the delegates in the list are 'checked', so that their calendar appears overlaid with mine. This happens often, and I cant see any pattern in which people's calendars are getting selected.
MacBook Pro with Retina display, OS X Mountain Lion (10.8.2)