If the number supplied to numBack exceeds the number of previously visited entries in the history, the System.Windows.Forms.WebBrowser control will navigate to the first item in the history, for example, to the first Uniform Resource Locator (URL) visited during the current browser session.
If there are no entries behind the current page in the navigation stack, calling HtmlHistory.Back(int) will have no effect, but it will not raise an error.