Continuity mistake: When Dark Helmet takes the ring from Lonestar, he throws down into a grate. Later, when Megamaid explodes and Lonestar and company cheer, for a brief second, you can see the ring back on his finger.
Continuity mistake: Spaceball-1 is using a blue tractor beam to bring in Vespa's car, but when Barf opens the roof to rescue the girls, the blue light is nowhere to be seen.
Continuity mistake: When Helmet is trying to hit Lone Starr with his Schwartz but Lone Starr is holding his head, you can easily tell that Helmet's Schwartz is made shorter so it won't hit Lone Starr.
Answer: "There are two sides to every sword" is a common saying.
Grumpy Scot