In ASP.NET 1.1, only one server-side form tag is allowed on a page. This error occurs if you have two or more server-side form tags on your page.
A server-side form tag is the tag which has a runat=”server” attribute. If this attribute is missing, then it’s a typical HTML form tag. The conclusion is that you are allowed to use multiple form tags on a page, as long as only one has the runat=”server” attribute. The disadvantage of the form that doesn’t have this attribute, is that view state won’t work (meaning form values will disappear when using the back/forward browser buttons). It’s a small price to pay if you really need multiple forms on a page.