Up until yesterday, I was still running Spanning Sync on Lion without any problems. I was also syncing my Mac with an iPhone, so apparently that was how it was continuing to work.
Anyway, I neglected to deactivate Spanning Sync before upgrading to Mountain Lion. Huge mistake. First thing, Spanning Sync came up with a warning that it wanted to delete hundreds of events. Naturally, I refused to let it do that until I could figure out what was going on. Once the dust had settled, I had hundreds of duplicated events on my local calendar (though thankfully not on Google Calendar). Still cleaning up the mess from that one.
Anyway, the takeaway from this is to either uninstall Spanning Sync before installing Mountain Lion, or at least turn off calendar syncing.