Very scarry situation. Some info first:

Server: CentOS release 5.6 (Final) 32-bit
Zimbra: Release 7.1.3_GA_3346.RHEL5_20110928134520 CentOS5 FOSS edition
OSX 10.7.2
iCal 5.0.1
Safari 5.1.1

One of my users complained about missing items in her calendar. Input is done with iCal and Zimbra as CalDAV account.

I checked this using her account on her iMac and used Safari to login to Zimbra.
A lot of items were stored in the trash. I checked them all and moved them back to her calendar.

Logout and login with the web gui, random items were back in the trash. I could reproduce this several times.

I switched to my account on that iMac, used Safari to login to Zimbra with her account, restored the items but could not reproduce the logout login effect anymore.

I switched back to her account on the iMac and was no longer able to reproduce this effect.

I even tried to delete items with iCal, web gui, anything. I can't reproduce this anymore.

Anyone seen this effect before? How can I be sure it won't happen again?