As Muttley is lowering Dastardly after the latest mishap, Dastardly tells him to slow up as they are 100 feet from the ground. Muttley says they are 500 feet from the ground (thanks to his glasses) and when he brings his hand up to show it, he gets a fifth digit.
Dastardly: Take over the controls Zilly
Zilly: (Seeing things upside down) Well its about time someone did. It looks to me like we're flying upside down
(Turns the plane upside down)
LOOK CLOSELY: As Muttley crashes to the ground with Dastardly in tow, a recalcitrant cel can be seen for a couple of frames.