Output error vs Compiler error

programming flash
1

In my current flash project I keep getting this error in my output panel:

TypeError: Error #1010: A term is undefined and has no properties.
    at main_flash_fla::Acoustic_1/moveImage()
    at main_flash_fla::Acoustic_1/on2()

However, It does not seem to effect the movie when I test it. All it has really accomplished is to annoy me by always popping up in my output panel.

Heres my question: Why does it choose to show up in my output as an error rather than my compiler error panel? Also, what then is the difference between an error in output vs compiler?

And how do I fix it? Here is my code:

import fl.transitions.Tween;
import fl.transitions.easing.*;
import fl.transitions.TweenEvent;

var item:Number;

function moveImage(item:Number):void {
    var leftPosition=-161;
    var i:Number;
    
    for (i=1; i<item; i++) {
        new Tween(this["section"+i],"x",Strong.easeOut,this["section"+i].x,leftPosition,1,true);
        leftPosition+=36.5;
    }
    
    var rightPosition=-11.5;
    
    for (i = item; i>=item && i<=4; i++) {
        new Tween(this["section"+i],"x",Strong.easeOut,this["section"+i].x,this["placeholder_"+i].x,1,true);
        rightPosition+=36.5;
    }
}

section1.addEventListener(MouseEvent.CLICK, on1);

function on1(event:MouseEvent):void {
    moveImage(1);
}

section2.addEventListener(MouseEvent.CLICK, on2);

function on2(event:MouseEvent):void {
    moveImage(2);
}

section3.addEventListener(MouseEvent.CLICK, on3);

function on3(event:MouseEvent):void {
    moveImage(3);
}