Random happening at work: a coworker was using a power supply, and noticed it made a jingling sound when moved. Huh. Turns out all the heatsink clips had either fractured and fallen off, or were cracked through and just floating there.
They appear to be galvanized steel, and I suppose succumbed to stress-corrosion cracking, or something like that.
Protip: make sure your clips are good quality material, preferably stainless steel (they often are!). Avoid using galvanized steel, it would seem.
Tim