I have also experienced this problem, when the users were also syncing their exchange calendars to a mobile device like a blackberry. One of the ways to avoid the problem is not using cached mode.
From Outlook select Tools, Email Accounts, View or change existing e-mail accounts. Select Exchange server, then click the Change button. There is a check box for Use Cached Exchange Mode, remove the selection and choose Next and Finish. Restart outlook and try again.
Hope that helps!
We also instruct our users to not change or modify any appoinments from their mobile devices. Always do this from the desktop client to avoid issues.