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iMac G5s of both sizes (shown) develop these cracks just sitting there on the shelf, with little UV exposure and only mild temperature fluctuation. Same with clamshell iBooks. These were the two that survived a shelving collapse years ago with no such injury!
It occurs to me this may be from moving them by holding the case by the side instead of the bottom, which is a thing I totally do. Whoops. Not that it should be so flimsy.