Circuit breaker wired incorrectly
This is dangerous and could start a fire!
If I understand you correctly, you have a circuit which contains a mixture of #12 and #14 wire, that is protected with a 20 A breaker. If so, that breaker needs to be changed immediately to a 15 A breaker. #14 wire is only rated for a 15 A breaker, and having any #14 wire at any point in the circuit, no matter where and no matter how little wire there actually is, means the circuit must be protected by a 15 A breaker. You must use the smallest current rating of any of the wires / devices in the circuit to determine the proper breaker size.
The breaker should be turned off and the circuit not used until it is replaced.