Sporadic coldfusion.runtime.CfJspPage$ArrayBoundException

I have the following loop, going through an array, and trimming the of element i and then overwriting the element with the trimmed value:

for(a=1; a <= ArrayLen(arguments.locationHeaders); a++){

     arguments.locationHeaders[a] = Trim(arguments.locationHeaders[a]);

}

Every so often I get an error stating The element at position X cannot be found. 

I'm looping through the very array I'm editing!  How can I be OOB...?

If I refresh the browser it works fine.