Uhhh... that does not sound familiar. Was it a draggable window with a title bar, or a message that was part of the page?

Did anything happen when you clicked the button (or used the X button on the titlebar) to close it?

Maybe it's a message generated by the web page when it receives some unexpected input. Just guessing.

Thank you. I contacted the website I was working on and actually recreated the same dialog box on Google Chrome. Its an error on their system.

Phew... Thank you for your help!