iOS 6 has a new feature where if you are using a shared calendar and someone else makes any change – creates a new event, moves and event, changes length of event, etc – then you will get an alert on your iPhone or iPad. Personally, I find this very annoying as often the people I share calendars with add or edit tens of events at a time and my iPhone just keeps beeping away like crazy.
I eventually figured out how to turn this alert off. Just go to the Settings app, scroll down and select “Mail, Contacts, Calendars”, then scroll down again until you see the “Shared Calendar Alerts” setting and just flick it off. Now your phone won’t have a fit when other people add or edit calendars.