Ardu Version: V3.2.1 Build: (36b405fb)
Ardu Type: APM:Copter
Hardware: 2.5
Frame Type: H
FAILSAFE SETTINGS:
Main Battery = Activated at 10.3 volts or 650 mAH, with RTL
Ground Control = Not Activated
Receiver = Activated at 950 PWM with always RTL
GEO Fence = Not Activated
This topic might be of interest to some of you that have receiver Fail-Safe parameter activated and to consider the following incident that occurred to me a few days ago. As required, I had FS trigger to a min of 10 PWM below the reading I was seeing while my TX throttle was at its lowest position ( 965), thus my trigger was set to 950. That evening on completion of RTL and the other during a normal stabilize landing, the motors would not completely power down, had erratic RPM and nearly flipped itself over, eventually system responded to my constant left yaw command and De-Armed itself.
Data indicated system had entered FS mode and attempted RTL because readings attained were 942 on one landing and 949 on the other; these were below set trigger of 950 and thus system proceeded to Fail-Safe mode.
Verified to see if I had earlier screwed up on what my lowest TX throttle position readings were; but no, I was getting anywhere from 962 to 965 when throttle was at its lowest position. Then I tried applying some slight pressure on throttle stick and then I was able to go as low as 942 
Conclusion, 
at the flying field we might unknowingly exert more pressure then usual on throttle stick compared to at home while performing radio calibration or setting up Fail-Safe trigger. I have now edited to trigger FS at 925 vice 950 PWM. Try it for yourself you will see different PWM results ( pending stick potentiometer play) if you casually bring down throttle sick to its lowest setting vice constantly holding slight pressure on stick.