Comments by mickfarrow (Top 1 by date)

Hi,
I had this problem a few weeks back also, it turns out that this behaviour is correct, when the page is posted back to the server it is expected to have the same controls present as when it was served up to the client. In your example you have used JQuery to construct a combo box, therefore the event validation fails as it detects the new control.
The easiest way to overcome this is by using server side code to populate the combo box. If you cannot do this you can turn off event validation, or use a method such as ajax instead of posting the form back.