It's not possible to modify the same event twice in a single KOrganizer session. You have to restart KOrganizer to be able to modify the event again. After clicking "OK" in the component editor after modifying the event for the second time, a dialog appears saying:

"Unable to store the incidence in the calendar. Try again? Reason: NO Only resources can modify remote revisions " [Yes] [No]

(Offtopic: there should probably be a newline between "Try again?" and "Reason...")

I /think/ the problem is that KOrganizer somehow caches remote revision of events, but does not update the cache when remove revision of an event changes (which happens for instance after editing an event in a Google calendar), so when user tries to edit the event second time, KOrganizer sets incorrect remote revision which looks Akonadi as if KOrganizer was trying to change it.

Reproducible: Always

Steps to Reproduce:
1. pick a random event in your calendar, using resource that uses remote revisions (for example the Google Calendars resource)
2. edit the event, confirm
3. edit the same event again, confirm
Actual Results:  
Error dialog appears.

Expected Results:  
Event is successfully modified