Inability of alkyl fluorides to be reduced to an alkane in the following reaction
Other answers may be more elegant, but it boils down to the carbon-fluorine bond being too strong, in fact stronger than carbon-carbon or carbon-hydrogen bonds.  The required bond breaking simply cannot be done under normal conditions.  Similarly Grignard reagents (formed with a much more powerful reducing agent) are much less common with fluorine than with heavier halogens, and the bad things chlorofluorocarbons do to our ozone layer come from breaking $\ce{C-Cl}$ rather rhan $\ce{C-F}$ bonds.