Revealing mistake: When Lowry first arrives at Buttle's flat to return the cheque, he hears a cough and turns to see a man smoking a cigarette, half hidden by a column. When he blows a cloud of smoke it is easy to see that it comes from a tube next to his mouth. He isn't even exhaling at the time.
Revealing mistake: When Sam and his mother are going through the restaurant's security checkpoint, the image seen on the X-ray machine shows the same collection of objects that are supposed to be in Mrs. Lowry's shopping bag before she places the bag into the machine and then again when the bag sets off the alarms.
Answer: The song "Brazil" represents a utopian world far off in another part of the world. Characters living in this dystopian world will sing "Brazil" as a form of escapism.