Star Trek IV: The Voyage Home - A Helping Hand....Well Finger.
About halfway through the movie when Sulu is piloting the helicopter over the bay with the plexi-glass underneath, he accidentally turns on the windscreen wiper. Look carefully at the bottom of the wiper. You can clearly see a finger moving the blade. This Slip-Up is mentioned in the 'text-commentary' on the special edition dvd. If you have trouble finding it, its in scene 13 at aproxx. 1 hour, 19 mins, 10 secs.
a widescreen version of the movie; DVD
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||The Finger - Magnified