There has been at least one report of a vehicle flying Copter-3.4-rc1 and disarming in the air when switching from AUTO to LAND mode. (see report here).
The cause is not yet clear but I recommend people be very cautious flying Copter-3.4-rc1 in fact, I would go so far as to recommend testers wait until we get to the bottom of this and release Copter-3.4-rc2 with a fix.
Thanks for the testing and patience.