Ever since upgrading to Mountain Lion (and I've done two clean installs and set up the Exchange account at least twice now according to my organization's settings, so I don't think I'm doing anything wrong), I've had problems with events on my Exchange account in Calendar giving the following error: Calendar can't save the event "event name" to the Exchange Server. The account "Exchange account name" currently can't be modified. To discard your changes and continue using the version of your calendars that's on the server, click Revert to Server. My options are Revert to Server, Try Again, and Ignore. I get this with lots of events, and I think I've narrowed it down to using the "Sleep" button on calendar alerts that pop up. From looking at the event, it appears Calendar is trying to add a second alert to the event (at the snooze time). Exchange supports only one alert per event, so I'm guessing this is what's causing the problem. Has anyone heard of this before or know a solution? The best thing I can do now is wait several minutes for Calendar to finish syncing with Exchange (and it freezes while that's happening!), then click "Revert to Server" and modify the event as I see fit when it's all OK again. As you can imagine, this is pretty annoying, and it seems that Mountain Lion's Calendar.app (almost didn't notice it changed from iCal) is a bit buggy. I find it hard to believe that I'm the only one with this problem, but I can't seem to find much with a search.