Many of you know this already, but please allow me to state a critical fact. Any wire carrying current will heat up. Fat wire will not heat up much. But copper is expensive, so extension cords with fat wire are expensive. A light-duty extension cord (16 gauge) running the 12A of the Leaf EVSE will heat up significantly. It isn't safe. Extension cords with 14 gauge wire are rated to 15 amps for runs up to 50 feet, but as someone mentioned, a 12 gauge extension cord is even better and gives you a little bit more safety margin.
Also, a worn out cord or outlet can heat up excessively, because the connection can have significant resistance. I found this myself when I plugged my EVSE into a home outlet and noticed that the outlet front was getting quite warm. I took off the plastic cover and measured that the outlet itself was 30 degrees hotter than the room air! So I replaced that with a new, quality outlet, and now I only see 6 degrees of outlet heating.
I doubt that the outlets on cheap extension cords are made well enough to operate safely at high current after being plugged and unplugged hundreds of times. And wear and tear can cause the connection inside the cord, between the wire and the outlet, to fray and raise resistance. So if you're buying a cord for frequent use, this may not be the time to be cheap.
For more detailed advice, see:
http://www.generalcable.com/NR/rdonlyres/5EF31BDA-6D5C-4599-90A6-1D6DC9F32CBC/0/Pg94_Wire_Gauge.pdf" onclick="window.open(this.href);return false;
Bob