Part of being a Road Scholar (or Roadgeek if you prefer) is a 6th sense about roads. I can often sense when something is different or not quite right.
This happened to me recently along US 71/I-540 between the Missouri State Line and Fort Smith. I wasn't sure what it was, at first, but something didn't feel right. Then one day, I did a mileage check: as I drove it, it was 97 miles from the Missouri State Line along US 71 to the Oklahoma State Line near Fort Smith along I-540 (technically, it's US 271 at the State Line, but it's a continuation of 540). This was as I drove it, following US 71's current alignment along I-540 and I-40. No big deal, right?
Now, it gets interesting: as 71 is signed, it is 100 miles from Missouri to Oklahoma. I base this off the mileposts and exit numbers along 71 and 540. The last posted exit is Exit 98 for AR 340 at Bella Vista. From there, it is two more miles to the Missouri State Line. So that means the exit number is wrong if you base it on how I drive it. The next numbered exit going south is Exit 93 for US 71B at Bentonville. It's also incorrect. My calculations show it is 7 miles from 71B to the Missouri State Line, so this should be exit 90, not 93.
Based on what I have found, the error occurs somewhere between US 62 (Exit 86) and 71B (Exit 93). I've not figured out at exactly what point the error occurs, but may be a matter of multiple round-off errors (rounding mileage up to the next Milepost). Either way, it probably doesn't make much difference and I'm probably the only person who has noticed this.
Someday, maybe I'll ask AHTD about it... but not today.