Try this on your phone:
Open the calendar, and make a meeting appointment (this may affect other types of appointments too, I haven’t tested)
Set the alarm “On” and hit “Done” to save it.
Now, wait until the appointment rolls past and the alarm sounds. Edit the entry and update the time of the appointment to some time in the future.
That works fine, go down to the Alarm section and turn the alarm on again. You’ll get an error message saying “Alarm already passed!” even though the time of the appointment has been updated! It won’t let you set an alarm.
It also happened to me with my 6120c with symbian 3rd edition :S