Nigel has the straight story. There was an accident (Brazil) where the spindle let go, after the steel spindle liner had let go. Without the liner, the spindle had extra motion in lead-lag and flapping that overstressed the nut area. Retaining the liner stopped that mode, but it turned out that the normal motions produced enough fatigue damage that a second similar accident occurred (Bristow, Aberdeen). At that point, we installed a redundant steel rod that compressed the entire spindle, and also served as a second load path. Crews were sent world-wide to install the kits at no cost. I lived through these events as S76 project pilot, and it was so very traumatic. Gerry Hardy was a friend, and quite a pilot. I still can feel the pain of that moment.
I cannot describe the way those of us who build these machines put all our best into the work, and how much of us is in the aircraft. Through success and failure, you can be sure there is not a drop of cynical attitude, no attempt to be "good enough" when safety is concerned. I met the engineer who signed off those lightning strike blades, he was devastated when that accident occurred, absolutely devastated.
Just as pilots know that each decision can reach life or death importance, the designers, builders and technicians who develop and test the aircraft feel the same sense of responsibility in their hearts.