menu error after onbeforeunload event

[larkin]

I have the following function on my onbeforeunload event:

onbeforeunload="return 'YOUR CHANGES WILL NOT BE SAVED!!'"

When I use my menu which I created from a third-party script, and select one of the menu item links, my onbeforeunload event fires, but if I 'cancel', it then gives me a run-time error which says 'unspecified error' and leads me to the last line of the following link (menu) code:

function GoTo(){
if(this.LinkTxt){
status='';
var HP=Nav4?this.LowLyr:this;
LowItem(HP);
this.LinkTxt.indexOf('javascript:')!=-1?eval(this.LinkTxt)cLoc.location.href=this.LinkTxt}}

Does anyone have suggestions what this 'unspecified error' might be or how I can diagnose it?

Thanks very much for help.