The opening voice-over by Kris Kristofferson, "If it sounds Spanish, man, that's what it is; it's a Spanish movie," echoes the spoken opening of his version of "Me and Bobby McGee," which he wrote in 1970: "If it sounds country, man, that's what it is; it's a country song."
(at around 6 mins) When Ferrel is hiding behind the rock, the background is a two-part screen and was not lined up straight. However this movie has a lot of deliberate mistakes. A very obvious painted background, not lined up straight, is just one of them.
When Onza is on the phone with the police officer he tells him keep the Americans away and hangs up the phone, in the next shot the conversation continues and he has the phone in his hand again.