engine and other failures
R22: alternator failure, various tach failures
F28A: stuck collective
B206B3: starter/generator shaft failure, T/R chips
C172: night engine failure
Re: the F28A tipping over forward - the Enstroms really like to rock forward on their oleos, so I agree that with all that inertia it's worth trying to get the ground speed really low. That's not really an option in the R22, but the stiff R22 gear will slide pretty well if the terrain is at all flat, so I would opt for some forward speed in the R22 unless I had a heck of a headwind. The R44 has enough inertia to get really slow touchdown speeds.
I was lucky in the C172 - I flew just a few inches under wires that I couldn't see in the dark - almost had a heart attack the next day when we went back to look at the aircraft. Owner bolted a new engine in and flew it out of the field himself.