To reproduce:

Add a RadTextBox and a RadButton to a form. On the Leave event of RadTextBox show a RadMessageBox. Start the application. Focus the textbox and click the button. The RadMessageBox will show, click the OK button and now you will see that the button is in its 'IsDefault' state (it is a bit more obvious with the Office2010 themes) and no other controls receive mouse input unless you click on the form.

Workaround:

Set the CausesValidation property of RadButton to false.