I have four theories:
1. He didn't notice that he clicked inferno when he was trying to put something different in, perhaps because his computer was freezing up during the trade, and then he did the trade without realizing inferno was in his box.
2. He traded away inferno for something that he didn't really want as much, and he has serious buyer's remorse.
3. He was teasing someone when he never intended to trade it, and mistakenly hit the button to confirm.
4. He was all set to gain another inferno in a favorable trade, and because of some mistake on his part, he didn't complete the trade.
That last one, btw, is how I "lost" a pair of fire walls recently. If it is option #1, jasonpwns, I want to spend an entire weekend trading different stuff with you and, you know, see where it goes.
[I see he explained it while I was typing this....
So now I add:
5. He included it as part of a multiple window trade for something better (green flips?) and the person disappeared before finishing the trade and including their supervaluable item/pixels]