I had a similar situation happen to me today. I added a new attendee to a
meeting appointment and clicked on save & close (as always). The window
came up as usual with the options to send to the new invitee/to all, etc. I
selected send to only new attendee. The difference here is that it sent out
1) a "cancellation" notice to everyone, then 2) a new meeting notice. The
bad thing about that is if the recipient accepted the "new" meeting notice
first then accepted the "cancellation" it took the meeting off of their
calendar.

One of my associates had the same problem yesterday; which leads me to
wonder if there was some type of universal update done for corporate
subscribers yesterday.