A fumble six is the occurrence in American Football when the offensive player fumbles the ball, and a defensive player picks up the ball and returns it for a touchdown. Hence the six, for a touchdown is worth six points.
Ex 1: Dude I destroyed you in Madden 12! I got like 5 fumble sixes!
Ex 2: The highlight of the player's career was when he got a fumble six.
Messing up when the stakes are of high importance, especially in an RVK flex game. Then proceeding to lose out on a significant gain (usually in terms of finances or LP points).
B: "Ja som hrával Zeda"
B: *Proceeds to fumble the bag on Zed*
V: "gg enemy Katarina 25 kills, that epic fail, go next"