A rule of thumb I go by is that things are usually engineered to handle double what they are spec'ed to. So a beam that breaks at 1000lbs would be spec'ed to handle a 500lb load. I wouldn't be suprised to see a #14 wire hold 30amps.
So why is hooking up a #14 to a 20amp breaker dangerous? Because you are decreasing the safety margin. Add NM run near a heater with a lot of insulation and a marginal breaker and then you might have a problem for example.
