Ruy Lopez gambit
This variation is called "The Fishing Pole," I think with the idea that the Ng4 is bait for a nasty trap. After 6 hxg4? hxg4, white's Nf3 is under serious threat; but more importantly, moving the Nf3 allows ...Qh4 followed by a quick mate.
I think this line may work well for black in a blitz situation against players who haven't seen it before, but the consensus is that declining the "bait" leads to a good game for white. See http://www.cec-services.com/050323.fishing.hook.pdf. As a general chess rule, if you can't figure out why your opponent is handing you material (as opposed to simply hanging a piece), you should probably say "no." In other words, don't get into a car with a stranger who's offering you a piece of candy.
8...Qh4 9. Qh3 (stopping the mate) Qxh3 10. gxh3 dxc6 and black has a double attack on white's h-pawn. So not only will black be a pawn up, but he will have ripped white's k-side protection to shreds.
And I think Nh2?? was only an illustration of the badness that can happen in this line if you're not careful.
Thoughts ?