Your experience is that the increase in resistance will melt the receptacle and conductor at under 20 amps?
with a loose connection & a load.. Absolutely!
I had a service call to a new home once.. Home was only about 3 years old... The customer almost had a fire in a kitchen receptacle.. I got there & the wall was seriously scorched, plastic 118N box melted like a shrinky-dink, recept looked like something in the back of my fireplace.. I closely examined the recept & noticed something peculiar... The terminal screws were still backed out to factory set!
the installer only wrapped the conductors around the screws not bothering to tighten them down!... I checked EVERY device in that house & found over half were that way!
The load on the circuit was a 15A 120V jacuzzi plugged in the WP recept outside, which fed through here & from a counter GFI.. (Funny, I thought you weren't allowed to put other loads on the kitchen counter circuits sans the dining room or a couple other kitchen loads).. I wish I had pics from that place.. I only had a few left in my Polaroid at the time & gave them to the customer..
That was an extreme case, but I come across them alot when people start using those little "plug in" heaters..
PS.. click here for more "burning fun" http://electrical-contractor.net/ubb/Forum4/HTML/000348.html