A bulgarian backfire is a Halo 2 term. It is when a player tried to stick another player with a plasma grenade from point blank range, and the plasma bounces off the player and sticks to the person who threw the grenade.
"D00d! Sum n00b stuk m3 wi7h a Bulgarian Backfire last night on Xb0x Live!!!!11!!!11 I wuz s00 pissx0r3d!!!111!!1!1