Probably someplace where I exited a freeway for gas or something and then got right back on. I've taken enough long road trips and random exits that the number of tiny segments of state routes I've obtained this way is not worth keeping track of, but I've been known to do this with US highways intersecting interstates simply to say I'd been on them. My very short mileage on US 43, 73, 81, 136, 159, 177, 266, and 611 all falls into that category.
Outside of stuff like that...I've clinched all 0.085 miles (450 feet) of unsigned Utah 231.