Supposedly, the red badge that shows up on the Calendar icon in the app switcher (and the Dock, if anyone still uses that) represents invitations that you've not dealt with. That's a nice theory, but I have no invitations waiting for me to act on, and I still have a red badge with a "3" in it.
I realize that I can disable the badge, but that seems like taking a hammer and smashing the light on the dashboard, instead of figuring out why it is lit up.
I want to know what (other than invitations waiting) could cause a red badge to show up on the Calendar.app icon. If this is a bug (e.g. where Calendar is failing update its count of invitations) is there any way to fix it? Is there any way to reset it? If it's not a bug, what other entities get counted besides invitations?