I remember one position where it said you sack your knight! I thought it was a crazy idea but there are chances of attack.
The knight sack you may be refering to may be the Halloween Gambit. Another fun one to play in blitz though a good player will beat it easily.
3.Bc4 is inferior to 3.Nf3, but the less people know that, the better. No need to reduce the client base without a good reason.