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A very common cause of strange behavior in scripts comes from not using the "==" comparison correctly. This simple script demonstrates a very common and time wasting scripting problem: 
var X =

     var X = 1;

...

     alert ("X is " + X); 
     if (X = 2)
     {
        alert("X is 2");
     }

...

If you execute this script this first alert message will show X is 1 and the second will show shows X is 2 … which is not possible.
The cause of this problem is of course that the if statement should have been 
if 
     if (X == 2)
     {
        alert("X is 2");
     }