this is most likely caused by:
1) faulty weedwacker
2) faulty extension cord
3) faulty electricity source (3 pronged outlet, but could it be badly grounded? how would I know?)
4) user error (I've used the weed wacker for similar amounts of time in the past, but maybe I somehow overdid it?)
also, if I scrub the charred stuff off the prong, is it safe to use the weedwacker again? Can one replace the weedwackers plug for less than the cost of a new weedwacker? it's less than 5 years old and is a black and decker, so I'm suprised to see it failing (for whatever reason).
in a possibly related occurance, I was vaccumming last week for about 2 hours (cat hair + drapes + rugs = several passes before a room is "done") and the vaccum got very hot, so I turned it off to cool down. 2 hours later I plugged it in and the switch would not turn the vaccuum on. I only mention this in case there's an obvious "OMG you have too much electricity coming out of the outlets and it will eventually destroy anything you plug in" explanation.