The question isn't whether it's "bad," but whether it is legal. Running a 20A branch circuit over #14 wire is not legal (there are very uncommon configurations that it is pointless to explore here).
If you have any questions at all, bring in an electrician.
If it isn't legal, there is a reason it isn't legal, and the reason generally boils down to fire safety. You could (illegally) push 50 Amps over a #14 wire and power a load, but the wire is going to get hot. The more continuous the load, the hotter the wire is going to get. Voltage drop due to resistance is probably going to damage the load (especially if it's a motor or electronics), and chances are good that you'll cook wire connectors and start a fire, which may burn your house down and kill everyone in their sleep.
The code errs on the side of safety. I've actually seen horribly incompetent cases where, for example, a 240V water heater on a 30A circuit (two pole 30A breaker) was being served with #14 wire and it had been working for years. But maybe the next family in that house taxes the water heater harder and the wire heats up enough to start a fire.
Did you know that the vinyl sheath on NM cable will burn and carry a flame right through the walls of your house (stopping that flame from propagating down the wire is one of the main purposes of NM cable staples).
The bottom line is that it is not legal to put a 20 Amp breaker on a #14 wire. You need to replace the wire, or replace the breaker with a 15A breaker.
The breaker protects the wire, and protecting the wire protects lives.