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