Something that didn't attract much attention on its release, largely because of the antics taking place on the Hudson River, was the Danish final report into the first of Scandinavian Airlines' three Q400 landing accidents - captured on video at
You can find all 142 pages of it here, but the abridged version is this: condensed water collected in the right-hand retraction actuator piston rod, leading to corrosion which eventually allowed the rod end to separate. The right-hand landing-gear free-fell into position and the resulting stress prevented it from locking in place.
Approved maintenance procedures contained no specific inspection requirement for the actuator or the rod end. "The aircraft maintenance records were in compliance with the established maintenance program," says the accident report. It adds that while the right gear collapsed, inspection of the left gear showed it wasn't looking too clever either.