Please vote for
Bug 62432 in Bugzilla.
I'm am also seeing this error under 10.7 Lion (retail). I can consistently reproduce it by doing the following:
Create a new event (by dragging or other means)
Double click the event to bring up the edit window.
Change the calendar (e.g. from home to work).
Click done.
Every single time. I actually saw this same problem with Snow Leopard as well, it would just silently revert your changes. I've learned to edit the description, press done, wait a second...and then move it to another calendar because anytime you do both Zimbra gets confused.