You know that old game, where a player challenges another if he can count to 11 using his fingers?
Example: Player A attempts to count to 11, starting from his very left, and moving to his rightmost finger (both hands). The max he goes up to is 10.
Player B starts counting using the left hand, and counts up to five. Then on the other side, he counts backwards, starting with 10, going down to 6 as he counts fingers backwards. He concludes that since 5+6 = 11, he has won the game.
What is the logical fallacy present in this? I can't seem to think of a specific name for this, and it's really bugging me.