In this note, we present a simple example showing that the tangential derivative of a continuous function can vanish everywhere along a smooth curve without implying that the function is constant along the curve. We also give some sufficient smoothness conditions on both elements that prevent the previous situation from happening.
Moonens, L. (2009). Mean value theorems for tangential derivatives. 33th Summer Symposium in Real Analysis, Durant, OK, USA. https://hdl.handle.net/2078.5/40028