Written in Stone
I love the expression rock solid because it implies permanence. Few natural materials are as durable as stone -- if it's built from that, you can trust it's here to last. And in an erratic world, stability is crucial unless of course there's been a mistake . . . and it's Written in Stone.
Inside the Westside Light Rail Tunnel at the Washington Park Station etched in granite on a 260-foot core sample of Portland, Oregon's West Hills, you will discover a 16-million year recitation of history which includes the first 107 digits of the transcendental constant pi. We (should) all remember pi as the ratio of the circumference of a circle to its diameter, and its digits never repeat or follow any pattern. Other than its role in geometry, pi seems to have no special significance (as far as I know!) still it has held the human imagination hostage for almost 4000 years starting with the Babylonians and Egyptians. Unlike all the other versions of pi publicly displayed on the planet, the engraving in Portland is unique for one special reason: it's incorrect after the 11th digit.
Of course, most people won't even notice the discrepancy or probably even care, but this erroneous engraving provides at least one valuable lesson: double check your facts before they're Written in Stone!