I must humbly admit I misspoke. I can not find any specific code references to mixed wire sizes within a branch circuit. The closest I see is a reference to NO provision allowing a smaller conductor as a switch loop in a 20 amp branch circuit.
Wiring simply must be sufficient for the ampacity of the circuit protection. Generally 14 awg is used on 15 amp circuits and 12 awg on 20 amp circuits.
A receptacle that is rated for 20 amps is not allowed on a 15 amp protected circuit, whereas a 15 amp receptacle is allowed on a 20 amp circuit.
I appoligize for my mistaken comment.
Thank you to those who corrected me for providing some good clear explainations to go along with the chastizing.