Some time ago I noticed that Microsoft Outlook recognizes calendar invitations received via email and, before being accepted or declined, automatically adds these entries to my calendar.  When I first experienced this, I thought it was a bit odd and accepted it as a feature.

Then I began to notice that external Outlook users (not in my organization) could add meetings to my calendar by sending the invitations through email.  I would occasionally receive some invitations from vendors and would need to decline the invitations to remove them from my calendar.  Still not a big deal, but a bit annoying.

I now rely on Outlook for keeping informed about my schedule and for maintaining a historical record of events that I've attended.  It's quite handy (but still doesn't compare to Google Calendar).

Well, a couple of weeks ago I was invited to and attended a meeting.  The meeting was scheduled using Outlook by someone from another organization.  At the conclusion of the meeting the participants decided to have another meeting later in the day.  I was surprised when I went back into my Outlook calendar and found the original meeting had disappeared and a new meeting was scheduled later that day.  I learned that the organizer had simply changed the time of the original invite and Outlook updated everyone's calendars by moving the entry to later in the day.  After this happened one more time with this organizer, I requested that she create new invitations and leave the existing invitations as a record of our past meetings.

I did a bit of research and was unable to find an option for disabling this feature of allowing others to alter my calendar entries.  This Outlook "feature" seems to be a vulnerability.  Here is an excerpt from Wikipedia's entry on Information security: 

In information security, integrity means that data cannot be modified without authorization.

Not only does Outlook allow non-authorized people to add entries to my calendar, but it also allows these same people to move and delete these entries from my calendar!

In my opinion, a far better design would be for Outlook to present to the calendar's owner any calendar changes submitted by others and then allow the owner to allow or deny additions or deletions, and in the case of rescheduled entries, to allow the user to maintain the existing entry and create a new entry.