DefaultSubmit behaving oddly

I am using VisualStudio 2008; code behind is vb.
Three 'forms' on the page:
Login in Search Register I have used the DefaultSubmit behavior successfully in a number of places. Here's where this one goes wrong:
Onfocus in the login form - enter key will fire the form.
Onfocus in the Register form - enter key fires the form.
Onfocus in the Search box - either the enter key or the submit button fires the Register form.
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