IT does occasionally happen, and if a unit does indeed cross there is a procedure to follow to alert the neighbor that an accidental incursion had occured. So as to avoid any confusion.
I know this personally. I was in the Honor Guard Unit stationed on Fort Huachuca in 1997. We were given directions for a funeral detail of an American soldier, we knew we were close to the border but thought we were okay. We went to the site carried out our detail, and went home. A day later we realized after looking at maps that we had to have been on the Mexican side of the border, equipped with dress blue uniforms and M-16 loaded with blank ammunition. We reported our incursion, that was unpleasant.
I do not believe the government of Mexico has reported their incursions properly, they deny any incursion despite eyewitness reports.
There is no doubt in my mind that accidental incursions occur.
Actually,what I was referring to is the mention that it could have been the Mexican military. The first report on Jan. 4 that I read from the Department of Homeland Security mentioned that the Mexicans were wearing military uniforms. That article has been pulled and I can't find it anymore. However, I am not the only one who saw it as I will refer you to someone else who saw the post and has information from the HS website.
http://www.freerepublic.com/focus/f-news/1762636/posts?page=124#124