Oh, another issue, and this is actually a pretty big one:
If your LAND_COL_MIN is set too high, such that the helicopter has trouble descending for a landing, then during an RTL or Land Mode, it can think that it has landed, and disarm in the air. I had a bad crash because of this. This also affects large efficient multirotors.
For helicopters, making sure you have sufficient negative pitch to descend will avoid the problem. In my case, I had set LAND_COL_MIN to give nearly zero pitch because I had a ground resonance problem. I fixed that with stiffer landing gear, but forgot to change this to a more reasonable negative pitch value (-3 or -4 degrees should be enough). During an RTL on a windy day, it disarmed and fell from 30 feet.
However, I’m working on a patch that will add a check for the Motor Interlock to be False before accepting a Land Complete. This is a double edged sword. It prevents any chance of a mid-air disarm because of a false-positive Land Complete. But it also means that after an RTL, etc. the copter will not disarm until the pilot drops the Motor Interlock. This will be an optional feature.