One neat thing about software like MediaWiki (which powers Wikipedia, among other sites), is that it keeps a record of every change that is made to a document. That way, it is easy to see what the history of changes has been and respond when information changes.
It seems to me like it would be very useful to have the same technology in my calendar. So often these days, things get moved around and re-scheduled. It would be useful if I could annotate my calendar to know what is certain and what is uncertain, which appointments have already been rescheduled, and so on.
It would also be useful for situations where something accidentally gets deleted. If I delete my only record of an event, the chances of me remembering and showing up are virtually nil. That is one reason why I maintain a paper copy of my calendar in a page-a-day Moleskine, in addition to the Google Calendar I update from computers and my phone.
This is why I changed from using an Excel spreadsheet to a Google Spreadsheet for my personal budget – Google Docs keeps such a record, so if I make an error in budgeting I can go back and find the exact entry that created the problem.