Em wrote:
I added 14-gauge (15A) romex cable to a circuit that had 12-gauge (20A)
wire, indicating I had made a mistake, but it turns out to be a 15A breaker
on the circuit with 12-gauge wire. Is that OK/safe? I don't have to correct
my mistake, right? Thanks

Correct. However, if the #12 wire goes to the panel you may end up
confusing the next person looking at the electrical system--they may
assume that since it's #12 wire its safe to put a 20A breaker on the
circuit, not realizing that the circuit includes some #14..

Chris