It’s always bothersome in any restoration that the most minor part can be fatigued to the point that its imminent failure is so easily overlooked, and so the wheels fall off literally in this case.Glad it survived.
There is a tool that can be used to check parts like that. It’s called Magnaflux. It’s sort of like an X-ray, and it shows stress lines in metal objects. When metal fatigues it weakens in several spots before it breaks, and Magnaflux will show them. I heard about this thing ages ago; it’s used by race engine builders to see if the crankshafts and connecting rods are up to the job when doing rebuilds. I have no idea what it costs, or how skillful the user has to be.