Star Trek: The Next Generation - Tricorder Mix-Up
When the doctor who lost her son in the attack of the crystalline entity, the one before Data was found, is in the cave scanning she is holding the tricorder wrong (the scanning apparatus is facing her body). She then goes on to say that she is reading something in the rock face. But the scanning apertures are facing her so they are reading her life signs and such not anything on the rock face in front of her. It would not be possible to scan the rock face with how she was holding it. then in a later scene when she puts it away it was being held the right way again.
a copy of the 2nd episode with the crystal entity