Looking at the photos I think the culprit was the extension cord, not the adapter. As cords get used, the contacts tend to loosen up. When you plug in to the cord there is a spring-loaded contact that is supposed to "grab" both sides of the male end of the plug. The spring is supplied by tension in the female receptacle. As the receptacle ages that spring tension weakens, and the contact area, which is small to start with, gets smaller. Eventually it becomes a resistive contact, and heats up, with these documented results. The solution is to use high-quality cords, and to replace them when it becomes real easy to plug things in to them.
Alan Hepburn - San Jose, Ca
2007 Bounder 35E being pushed by a 2008 smart fortwo Passion
Good Sam Member #566004 * FMCA Member #F431612