This has been discussed previously here:
My opinion remains that even if there is a listing issue with minimum 14 AWG conductor rating on the breaker, the reality is that a 16AWG conductor is held very securely by every 15A molded case circuit breaker I’ve ever seen. And even if a loose connection were developed, there is insufficient current draw to cause any appreciable resistive heating and certainly nowhere near enough to cause an arc to be struck.
Now, double-lugging a 16AWG with another larger current carrying conductor in an inappropriate terminal could get one into trouble. But in my experience, even in this case resistive heating of the terminal will cause the thermal mechanism of the circuit breaker to trip before it starts melting the cable assembly.