The state line here is a river that will not hold still. The Red River meanders across a wide sandy bed and shifts its channel in floods, which left the Texas-Oklahoma boundary genuinely uncertain for most of a century and produced a Supreme Court case, an oil boom on a disputed sandbar, and in 1931 an actual armed standoff between the two states' governors over a bridge. Texas and Oklahoma finally fixed the line by compact in 2000, using the vegetation line on the south bank rather than the water.
Getting there
33.7180, -97.1520
Locations are approximate and may be inaccurate, on private land, or otherwise inaccessible. Check before you travel.