Just last week I got to tear out some station wire that was connected to a receptacle in the living room and run outside on the exterior siding to a sensor flood light with no switch. Pretty obvious that the former homeowner did his own work here. Oh by the way this genious twisted one pair together and used them to counter clockwise wrap around the ground screw. Guess he knew the importence of grounding!
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.
#14 IS rated for 20Amps with 60 and 75 degree rated insulation. 25 Amps with 90 degree insualation per Table 310.16. So yes, #14 will handle 20 Amps but code does not allow fusing at graeter than 15Amps