A Christmas Story Gestural Slip?
Thanks to Jeff for the link!
In the clip below, we see the character Ralphie lie about not not knowing where Flick is when he is asked by the teacher. When the teacher asks Ralphie if he knows where Flick kid is, you can see a single head nod yes before he begins to shake his head no. The slip occurs at about 12-13 seconds in the video at the link below.
Could this be a potential slip? Its difficult to know, since these are actors. However, if this occurred in real life, it could potentially be a hot spot where more questions may need to be asked.