I had a user report a very similar issue today. He's using Outlook and has a Blackberry synced through the Blackberry Connector. I grepped through the logs and found this activity happening around the time the appointment was created. Of special interest is the "Deleting appointment" event in the logs. Does this shed any light on the issue?
2009-10-28 08:48:00,692 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - SyncRequest
2009-10-28 08:48:01,070 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - SetAppointmentRequest
2009-10-28 08:48:01,074 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] mailop - moving Appointment (id=13069) to Folder Calendar (id=10)
2009-10-28 08:48:01,138 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - ItemActionRequest
2009-10-28 08:48:01,138 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] mailop - Deleting Appointment (id=13069).
2009-10-28 08:48:01,187 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - ItemActionRequest
2009-10-28 08:48:01,222 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - SyncRequest
2009-10-28 08:48:01,539 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=Zimbra-ZCO/6.0.5691.1 (5.1.2600 Service Pack 3;; en-US);] mailbox - FileUploadServlet received Upload: { accountId=d073510a-3c55-4131-b8f3-9e15ee90aa02, time=Wed Oct 28 08:48:01 MDT 2009, uploadId=b98c8865-40b8-4242-8840-8473cf80c202:2c2f13f2-f7cc-4e19-84bb-1e2d87bf7f2d, lslib32.bin}
2009-10-28 08:48:01,543 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - SendMsgRequest
2009-10-28 08:48:01,547 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] smtp - Sending message: Message-ID=, uploads=[Upload: { accountId=d073510a-3c55-4131-b8f3-9e15ee90aa02, time=Wed Oct 28 08:48:01 MDT 2009, uploadId=b98c8865-40b8-4242-8840-8473cf80c202:2c2f13f2-f7cc-4e19-84bb-1e2d87bf7f2d, lslib32.bin}], replyType=r
2009-10-28 08:48:01,688 INFO [LmtpServer-34365] [ip=xx.xx.xx.xx1;] lmtp - Delivering message: size=4944 bytes, nrcpts=3, sender=
ouruser@ourdomain.com, msgid=
2009-10-28 08:48:01,788 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] mailbox - IndexDeferredItems(null, 36860): Deferred count out of sync - found=6 in progress=0 (deferred count=9)
2009-10-28 08:48:01,867 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] mailbox - Deferred Indexing: submitted 6 items in 80ms (75.00/sec). (0 items failed to index). IndexDeferredCount now at 6 NumNotSubmitted= 0
2009-10-28 08:48:02,093 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] gal - autocomplete: overall=489ms, ranking=0ms, folder=438ms, gal=51ms
2009-10-28 08:48:02,244 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] mailop - adding contact
snowbird@ourdomain.com: id=13074, folderId=13, folderName=Emailed Contacts.
2009-10-28 08:48:02,324 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;aname=
zberryadmin@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForBES/5.0.2683.13;] soap - SyncRequest
2009-10-28 08:48:02,370 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;aname=
zberryadmin@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForBES/5.0.2683.13;] soap - GetContactsRequest
2009-10-28 08:48:02,466 WARN [LmtpServer-34366] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;] calendar - Unknown calendar item UID 040000008200E00074C5B7101A82E00800000000D089844AA957CA01000000000000000010000000B589C8E3E5C94245A42C5441F49EB39D in mailbox 155
2009-10-28 08:48:02,468 INFO [LmtpServer-34366] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;] mailop - Adding Message: id=13075, Message-ID=, parentId=-1, folderId=2, folderName=Inbox.
2009-10-28 08:48:02,527 INFO [LmtpServer-34366] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;] mailbox - outofoffice not sent (until date reached) mid=13075 rcpt='
ouruser@ourdomain.com'
2009-10-28 08:48:03,070 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - SyncRequest
2009-10-28 08:48:03,140 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - GetContactsRequest
2009-10-28 08:48:03,490 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=Zimbra-ZCO/6.0.5691.1 (5.1.2600 Service Pack 3;; en-US);] mailbox - FileUploadServlet received Upload: { accountId=d073510a-3c55-4131-b8f3-9e15ee90aa02, time=Wed Oct 28 08:48:03 MDT 2009, uploadId=b98c8865-40b8-4242-8840-8473cf80c202:1d97bf32-8c9f-494e-9658-a8beaf0c2cba, lslib32.bin}
2009-10-28 08:48:03,497 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - AddMsgRequest
2009-10-28 08:48:03,561 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] mailop - Adding Message: id=13076, Message-ID=, parentId=-1, folderId=5, folderName=Sent.
2009-10-28 08:48:03,621 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - SyncRequest
2009-10-28 08:48:05,561 INFO [btpool0-13765] [name=
ouruser@ourdomain.com;aname=
zberryadmin@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForBES/5.0.2683.13;] soap - SyncRequest
2009-10-28 08:48:26,940 WARN [LmtpServer-34369] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;] calendar - Unknown calendar item UID 040000008200E00074C5B7101A82E00800000000D089844AA957CA01000000000000000010000000B589C8E3E5C94245A42C5441F49EB39D in mailbox 155
2009-10-28 08:48:26,940 INFO [LmtpServer-34369] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;] mailop - Adding Message: id=13077, Message-ID=, parentId=-1, folderId=2, folderName=Inbox.
2009-10-28 08:48:27,036 INFO [LmtpServer-34369] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;] mailbox - outofoffice not sent (until date reached) mid=13077 rcpt='
ouruser@ourdomain.com'
2009-10-28 08:48:27,256 INFO [btpool0-13762] [name=
ouruser@ourdomain.com;aname=
zberryadmin@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForBES/5.0.2683.13;] soap - SyncRequest
2009-10-28 08:49:00,793 INFO [btpool0-13745] [name=
ouruser@ourdomain.com;mid=155;ip=xx.xx.xx.xx;ua=ZimbraConnectorForOutlook/6.0.5691.1;] soap - SyncRequest
"zberryadmin" is the Blackberry Connector user.