When I was young, my family took a number of cross country driving trips; we saw most of the natural wonders of the American west, to which I paid as much attention as any 14-year old boy with his nose in a book. We saw a lot of other things, as well - many beautiful, many boring, and some so ridiculous as to be sublime.
One particular trip took us through Utah, and so of course we made the mandatory pilgrimages to Bryce Canyon and Zion National Park. The main highway in the area is US 89, which we trusted to carry us safely through the area, and out once we were done. In the middle of a small town, though, the highway made a sudden (and poorly marked) left turn; we blithely continued on our way.
Clearly, we were not the first, for on the side of the road stood a tall, stately US Department of Transportation sign, crisp black text on a white background:
THIS IS NOT US 89
It was the least helpful sign we'd ever seen, and yet it was the most helpful sign we'd ever seen. We had no idea where we were, but we knew where we were not. It was glorious.
Ordinarily, I hate external links, but I'll make an exception here. As of December 2001, there's a photo of said sign available at http://carcino.gen.nz/images/index.php/5922d576/29d4a3f3 - use it wisely. If anyone knows the name of the town, please inform me - I think it was Panguitch, but I'm not sure.