It seems that once a landing failsafe sets in, the autopilot should not enter other modes, especially after landing! The following scenario unfolded about a week ago:
Takeoff manually in stabilize, go in loiter mode, battery failsafe incorrectly triggered (due to incorrect voltage readings), the copter comes down and lands; knowing that the battery is actually OK, I put it back in stabilize, then loiter then auto - the copter leaves on its mission, but it immediately starts to lose altitude (apparently still in land mode) while moving on the X and Y toward its waypoint. I get it back in stabilize mode and make a mini-crash-landing (my fault).
I don't think that the copter should be in landing mode and auto-mode at the same time. Please correct me if I am wrong, but I think that one mode switch should clear the previous one.