Doug...
I expressly note & advise my customers that if they choose to keep their existing aluminum feeder conductors (if they're salvageable)when I do a subpanel changeout, their warranty goes from our company standard for this type of work (10 years) to miniumum allowed by CA contractors law (1 year)
50% of the subpanels I've had to changeout over the years have been from a loosened aluminum connection similar to the above if I hadn't found it for another few weeks or months.. The subpanel above was saveable with some time getting to know a wire brush & some Noalox... Usually I find them with a phase in full melt-down, insulators melted, lug frozen from the arcing & heat...
The other 50% are Zinsco.. need I say more? [Linked Image]

-Randy