The URL to my test pages is here:
The validation was CLIENT side.
I worked again with this page and think that the problem is how the forms are specified. You are using ".forms" in the validation code in the FORM tag. But the table that was added was ABOVE that form. So when the code is re-examined upon re-opening the page the position of the form in the page hierarchy has gone from ".forms" to ".forms". I am guessing that your code in the extension does not take this into account?