Actionscript Code:
// button presses
upKey = 38;
downKey = 40;
leftKey = 37;
rightKey = 39;
focusKey = 65;
attack1Key = 83;
attack2Key = 68;
bombKey = 81;
autoAttack1Key = 87;
autoAttack2Key = 69;
trace("durr");
var listener = new Object();
listener.onKeyDown = function() {
    var key = Key.getCode();
    trace(key);
    switch (key) {
    case upKey :
        upDown = true;
        downDown = false;
        trace("banana");
    case downKey :
        downDown = true;
        upDown = false;
    case leftKey :
        leftDown = true;
        rightDown = false;
    case rightKey :
        rightDown = true;
        leftDown = false;
    case focusKey :
        focus = true;
    case attack1Key :
        attack1 = true;
    case attack2Key :
        attack2 = true;
    case bombKey :
        bomb = true;
    case autoAttack1Key :
        if (autoAttack1 == true) {
            autoAttack1 = false;
        }
        if (autoAttack1 == false) {
            autoAttack1 = true;
        }
    case autoAttack2Key :
        if (autoAttack2 == true) {
            autoAttack2 = false;
        }
        if (autoAttack2 == false) {
            autoAttack2 = true;
        }
    }
};
listener.onKeyUp = function() {
    var key2 = Key.getCode();
    switch (key2) {
    case upKey :
        upDown = false;
    case downKey :
        downDown = false;
    case leftKey :
        leftDown = false;
    case rightKey :
        rightDown = false;
    case focusKey :
        focus = false;
    case attack1Key :
        attack1 = false;
    case attack2Key :
        attack2 = false;
    case bombKey :
        bomb = false;
    }
};
Key.addListener(listener);
onEnterFrame = function () {
    if (upDown == true) {
        player._y -= 10;
    }
    if (downDown == true) {
        player._y += 10;
    }
    if (leftDown == true) {
        player._x -= 10;
    }
    if (rightDown == true) {
        player._x += 10;
    }
};

Rather than affecting one variable, one key press makes all the others true as well