Would appreciate it if anyone else can verify the following:

I can consistently recreate this in the ZWC 6.0.2 with IE 7 and Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729):

Creating a new calendar appointment: If you add a list of attendees either from the To: field on the Appointment Details tab or line-by-line on the Schedule tab, and then while on the schedule tab, you delete any one of the attendees by highlighting the name and using right-click to delete, then the following will happen:
1) You will only be able to add one more attendee to the list while on the schedule tab
2) You will not be able to see that attendee’s free/busy while on the schedule tab
3) You will not be able to add additional attendees while on the schedule tab
4) Once you click over to the appointment details tab, the last attendee you tried to add is lost.

Workaround: End-users must be told NOT to add/remove attendees while on the Schedule tab. They should only add/remove attendees from the To: field of the Appointment Details tab or the Attendees for this appointment field on the Find Attendees tab, and then keep clicking back over to the Schedule tab if they want to view free/busy.

Also added to bugzilla:

Bug 43228 – Web client calendar appointment attendee add/remove from Schedule tab causes problems