Wanna bet? The Mexican army comes across regularly. A few year ago, there was a quiet stink when a convoy drove across a bridge spanning the Rio Grande River and they played like they didn’t know they were in the US.
Somewhere at Google there is a map of all the fencing along the US/Mex border. Especially in Texas where the Rio Grand has many bends and loops, a lot of fencing is from the shortest point to the shortest point on the inside loop, so large chunks of unfenced land loops surrounded by river are easily reached by crossing the river and actually being in US territory.