Way back when I first started developing web applications, I quickly determined that you had to have the default button as the first submit button in the HTML markup.
A recent trouble ticket at work detailed that the default button was behaving erratically. It seemed that it was working only every other time. Yet, the location of the button never changed, so how could that be?
Here is a link of an example where I have recreated the issue.
Broken Default Button
To recreate the broken behavior, set the focus in one of the textboxes or a radiobuton and then press the enter key. The panel should minimize and a second panel should now become visible. click on the button in the upper right corner to re-expand the original panel and repeat the test.
every time both panels are visible, the default button switches to the button in the lower panel.
The fix is quite simple. Using asp.net 2.0 or higher simply add this one line to the Page_Load function: Page.Form.DefaultButton = "Button ID";
Fixed Default Button