Continuity mistake: On the truck cab's rear window, there is some sort of air conditioning unit installed. In some shots, such as the gas station scene where David Mann is on the telephone, or the railroad crossing scene, the air conditioning unit has a support structure to hold it in place. In the rest of the film however, there is not any support structure to be seen.
Continuity mistake: When Willard goes to confront his boss, Al Martin, about killing Socrates earlier that day, we get a good view all around Al Martin's office. There are a few rats on the desk, coffee table and a chair; but there are absolutely no rats climbing the curtains. Al Martin realises Willard is insane and they struggle briefly, whereupon a single rat leaps on Martin's arm and bites him as he screams. In that moment, seven rats suddenly appear climbing high up on the curtains behind Al Martin. The camera makes a one-second cut to Willard for his famous line, "Tear him up!" The camera immediately cuts back to Al Martin, and now all the rats have disappeared from the curtains in the background.