I have a feeling we may already have a thread to catalogue relevant examples, but it is not coming up in forum search, so I'm starting a new one for an example I spotted earlier this afternoon:
13 MPH speed limit sign (on private property, so MUTCD does not apply) just off US 160/K-2 on western fringe of Harper, Kansas (https://www.google.com/maps/@37.279167,-98.03954,3a,75y,133.29h,77.28t/data=!3m6!1e1!3m4!1sRjEz5RE3YAyfACxmblnb8g!2e0!7i13312!8i6656!6m1!1e1)
Yes there is another thread: https://www.aaroads.com/forum/index.php?topic=11967.0