What is this, and why is it happening?

Question

So I have been working on my final project for the semester for my Computer Systems class, and we have been tasked to make a game using Flash and ActionScript 3.0. I've pretty much completed everything but I have come across an extremely strange occurrence of Flash CS6 being silly. I am moving a MovieClip up and down depending on a selected index, however, the image leaves 'residue of it's footprints' behind and also moves. I have tried hard to look for an answer as to why this is happening, but I don't know what it's called, or how to appropriately explain it - I'm coming from an area where one must program graphics, not just simply, drag and drop.

Below are pictures as to what I've come across, but first the code I'm using:

function updateThemeScreen():void {
    button_selection.y += (selectedPositions[selectedTheme] - button_selection.y) / 2;
}
function attemptThemeChange(mxP:Number, myP:Number):void {
    if(objectContains(theme_darkness, mxP, myP)) {
        selectedTheme = 0;
    } else if(objectContains(theme_halloween, mxP, myP)) {
        selectedTheme = 1;
    }
}

As you can see in the final image, it has copied half of itself and left it at the last button, which is strange, and shouldn't happen...

Link to the SWF zipped up with the required AS3 classes: Dude, RUN

Answer 1

So it seems that I have magically fixed this problem by hiding the buttons and showing them all in one frame. I don't know what this problem is or why it does it, but to fix it, you just need to hide and show the affected components - yes, in one frame:

function hideShow(object:MovieClip):void {
  object.visible = false;
  object.visible = true;
}