I had an outlet that had plastic socket covers inserted for child-proofing. Recently I tried to removed the top cover and noticed that was quite stuck. When I eventually forcing it out, I saw burn marks on (what was left of) the plastic prongs. After I removed the outlet, it seemed evident that the top receptacle probably arced sometime in the past and melted the socket cover, part of the outlet housing, and the insulation around the hot wire.
However, this outlet is on a circuit with an AFCI breaker. Shouldn't the AFCI breaker have prevented this? Or is something else happening? I have tried the test button on my AFCI breaker, and it switched to a middle position that required me to turn it the off position before I could turn it back on.