Continuity mistake: The head of Mega-Maid mysteriously shrinks in size. After Spaceball-1 transforms into Mega-Maid, the head is gigantic enough for a Winnebago to fly through for a considerable length of time. However, later on in the film, after the head has been jettisoned and crashed on a beach, the head is only big enough to hold a few dozen people. This can be determined by the size of the people coming out of her nose.
Revealing mistake: When the "Alien" is dancing along the bar, it kicks its legs. As it dances past Barf and Lone Starr in close up, its legs are stationary and the whole model is bouncing. On cut back to its disappearance through the "stage door", its legs are kicking again. This also applies to the other animations of the "Alien." You can also see the 'stick' that it is attached to, along with the hole cut in the bar to accommodate the stick.
Continuity mistake: During the scene where the princess' Mercedes is being viewed on radar, you see that the ship is headed toward Spaceball 1; however, in the next scene the princess is moving away from Spaceball 1.