ModalPopup extender is a very convenient and easy tool for designing lightbox style modal popups.
During development, I realized that the contents that go in the modal popup box appear for a split second when the page gets loaded. In repeated tries, I was actually able to grab a screen shot of the strange behaviour.
Look at the contents below the button! They are a part of the modal popup contents. Pretty weird, huh?
To get around this - simply set the popup panel's style to display:none.
And the modal popup extender code
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