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
iMac:
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?