User Agent: Mozilla/5.0 (Windows NT 5.1; rv:10.0.2) Gecko/20100101 Firefox/10.0.2 Build ID: 20120215223356 Steps to reproduce: The first calendar in my calendars list is A. I create an event in calendar C. Both calendars are synched to distant ics (Google calendars). Actual results: Few seconds after event is saved, it is automatically reassigned to calendar A (with alarm in addition). If I change the event calendar back to C, sometimes the same bug occur (calendar swap), sometimes the event disappears (even worse). Expected results: You can guess, if I create an event to calendar C, I expect it to remain C. This bug appeared with version 1.2.x (currently 1.2.3). It is absolutely critical : we use Lightning with a specific calendar (not shared) for corporate reporting. With this bug, Lightning is totally unusable, we have to move back to old Excel report sheets (and forget about Lightning).