Errors: Strange behavior in scripts

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");
     }