I almost forgot about this one.
Last year, we switched from Netscape Calendar to Microsoft Outlook for our shared scheduling system.
I was pretty late to use Netscape Calendar -- I was always surprised how irritated it made people when we were trying to set up a meeting and I (among others) said "I'm not in Calendar." Surprised, that is, until I started using it.
Anyway, we switched over to Outlook. I had problems from the start -- no one ever seemed to get the meeting invitations I'd set up, nor the responses I'd sent. The helpdesk folks couldn't seem to help me, so I just sort of lived with it.
This went on for the better part of year. I finally got tired of it, so I gave the helpdesk another call.
The reason why my meeting invitations and such weren't getting out to other people was that I wasn't sending them -- I'd never checked the box to automatically send stuff. It was all piling up in my Outbox.
Now, with Netscape Calendar, all appointments and such were automatically sent to the server. Not so with Outlook, which I eventually figured out is essentially just a fancy layer over e-mail.
I hadn't had a lot of dealings with Outlook, because we use a rather different e-mail client, and in my personal computing, I was mostly Mac, and tended to stay away from Outlook because of its reputation as a virus/worm transmission platform.
Anyway, I checked the right box, and the messages from a better part of a year -- acknowledgments and invitations for long-past meetings, over 120 of them -- all went out in a big long whoosh.