Continuity mistake: When Cliff is being chased by the mobsters in the fancy restaurant, he escapes by flying through a stained glass dome directly above the dance floor. Two mobsters below aim up at him and start firing as he crashes through the dome, but no glass showers down on them.
Continuity mistake: When Cliff and Peevey first test the rocket pack by strapping it to a statue, the statue skids across the ground and crashes into a small mound of dirt on the otherwise completely flat field. But when they run over to collect the pack, the mound is gone and the statue is lying in a crater, as if it had hit the ground from a much steeper angle.
Continuity mistake: When Cliff and Peevy anchored the statue into the ground in the field to watch the rocket in action, they use a wired switch to activate the rocket. However, there's something wrong with this: there is no wire visible leading to the rocket/statue.
Continuity mistake: When Cliff straps the rocket on his back to save the pilot, he leave some of his coat's buttons undone, but when he lands in the pond, he's completely buttoned.
Continuity mistake: When he first flies the GeeBee, the joystick changes colors in the different scenes.
Continuity mistake: When Cliff first puts the Rocket on to save Malcolm, he doesn't do his helmet strap up. When they walk outside, just before Cliff takes off he turns back round to Peabody and his strap is done up.
Continuity mistake: When Neville Sinclair walks Jenny to the dance floor, the bandstand is empty. When we next see it, about 25 seconds later, all the band members are back. (01:01:53)
Continuity mistake: During the car chase at the start of the movie, the rear view mirror of the FBI car swaps from skewed to straight between shots.
Continuity mistake: The gum that Peevy sticks on the middle of the rocket changes position through the film.
Answer: The tether was designed to allow Lothar to be lowered from the zepellin and recovered later without requiring it to land (very tricky for an airship) hence it was hundreds of feet long.