Revealing mistake: When we see Dark Helmets' hoverjeep in the desert, watch the front end: you can clearly make out the edge of the mirrored box they used for the "hover" effect.
Character mistake: During the scene in the diner, there is a sign that reads "EMPLOYEE'S ONLY." There shouldn't be an apostrophe.
Answer: "There are two sides to every sword" is a common saying.
Grumpy Scot