The teardrop tattoo on el Casper's right eye is missing in two consecutive scenes on the top of the train but is visible on his face throughout the movie both before and after these scenes on the train. Interestingly, the tattoo is an important identifying mark/symbol in the movie and is specifically highlighted by gang members when asking locals if they have seen Casper as they try to find him and hunt him down.
Towards the end of the film, the city labeled as "Reynosa, Tamaulipas" is actually "Monterrey, Nuevo Leon."
Casper's clique is situated in Mexico (Tapachula, Chiapas), and yet most of its members speak with a Salvadorean accent and dialect (i.e. using voseo).
This is not only limited to MS13 members, since Martha Marlen (a normal civilian) speaks like that as well.